Light-emitting element

ABSTRACT

To provide a light-emitting element which uses a fluorescent material as a light-emitting substance and has higher luminous efficiency. To provide a light-emitting element which includes a mixture of a thermally activated delayed fluorescent substance and a fluorescent material. By making the emission spectrum of the thermally activated delayed fluorescent substance overlap with an absorption band on the longest wavelength side in absorption by the fluorescent material in an S1 level of the fluorescent material, energy at an S1 level of the thermally activated delayed fluorescent substance can be transferred to the S1 of the fluorescent material. Alternatively, it is also possible that the S1 of the thermally activated delayed fluorescent substance is generated from part of the energy of a T1 level of the thermally activated delayed fluorescent substance, and is transferred to the S1 of the fluorescent material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/944,257, filed Jul. 31, 2020, now allowed, which is a continuation ofU.S. application Ser. No. 16/412,529, filed May 15, 2019, now U.S. Pat.No. 10,734,594, which is a continuation of U.S. application Ser. No.15/946,094, filed Apr. 5, 2018, now U.S. Pat. No. 10,644,254, which is acontinuation of U.S. application Ser. No. 15/412,515, filed Jan. 23,2017, now U.S. Pat. No. 9,947,885, which is a continuation of U.S.application Ser. No. 15/051,910, filed Feb. 24, 2016, now U.S. Pat. No.9,559,313, which is a continuation of U.S. application Ser. No.13/957,612, filed Aug. 2, 2013, now U.S. Pat. No. 9,276,228, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2012-172830 on Aug. 3, 2012, all of which are incorporated byreference.

1. FIELD OF THE INVENTION

The present invention relates to a light-emitting element which includesan organic compound as a light-emitting substance.

2. DESCRIPTION OF THE RELATED ART

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence (EL). Ina basic structure of such a light-emitting element, a layer containing alight-emitting substance (an EL layer) is interposed between a pair ofelectrodes. By applying voltage to this element, light emission from thelight-emitting substance can be obtained.

Since such a light-emitting element is of self-light-emitting type, thelight-emitting element has advantages over a liquid crystal display inthat visibility of pixels is high, backlight is not required, and so onand is therefore suitable as flat panel display elements. In addition,it is also a great advantage that a display including such alight-emitting element can be manufactured as a thin and lightweightdisplay. Furthermore, very high speed response is also one of thefeatures of such an element.

Since a light-emitting layer of such a light-emitting element can beformed in the form of a film, planar light emission can be achieved.Therefore, large-area light sources can be easily formed. This featureis difficult to obtain with point light sources typified by incandescentlamps and LEDs or linear light sources typified by fluorescent lamps.Thus, light-emitting elements also have great potential as planar lightsources which can be applied to lighting devices and the like.

In the case of an organic EL element in which an organic compound isused as the light-emitting substance and an EL layer containing thelight-emitting substance is provided between a pair of electrodes,application of a voltage between the pair of electrodes causes injectionof electrons from the cathode and holes from the anode into the EL layerhaving a light-emitting property, and thus a current flows. Byrecombination of the injected electrons and holes, the organic compoundhaving a light-emitting property is put in an excited state to providelight emission.

The excited state of an organic compound can be a singlet excited stateor a triplet excited state, and light emission from the singlet excitedstate (S₁) is referred to as fluorescence, and light emission from thetriplet excited state (T₁) is referred to as phosphorescence. Thestatistical generation ratio of the excited states in the light-emittingelement is considered to be S₁:T₁=1:3. Therefore, a light-emittingelement including a phosphorescent compound capable of converting thetriplet excited state into light emission has been actively developed inrecent years.

However, most phosphorescent compounds currently available are complexescontaining a rare metal such as iridium as a central metal, which raisesconcern about the cost and the stability of supply.

Therefore, as materials which do not contain a rare metal and canconvert part of a triplet excited state into light emission, materialsemitting delayed fluorescence have been studied. In the materialsemitting delayed fluorescence, a singlet excited state is generated froma triplet excited state by reverse intersystem crossing, and the singletexcited state is converted into light emission.

Patent Documents 1 and 2 disclose a material emitting thermallyactivated delayed fluorescence (TADF).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2004-241374-   [Patent Document 2] Japanese Published Patent Application No.    2006-24830

SUMMARY OF THE INVENTION

In order to increase the luminous efficiency of the light-emittingelement, it is important not only to generate a singlet excited statefrom a triplet excited state but also to obtain light emissionefficiently from the singlet excited state, that is, to increase thefluorescence quantum efficiency. Thus, in a structure in the abovepatent document 1 or the like, in order to further increase the luminousefficiency, a material which emits TADF and has high fluorescencequantum yield is needed; however, it is very difficult to make such amaterial which satisfies the two conditions at the same time.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting element having higher luminousefficiency in which a material which emits fluorescence (hereinafter,referred to as a fluorescent material) is used as a light-emittingsubstance.

In order to achieve the object, one embodiment of the present inventionincludes a material for generating a singlet excited state from atriplet excited state and another material for obtaining light emissionefficiently from the singlet excited state.

Specifically, for a light-emitting layer, a material which can generatea singlet excited state from a triplet excited state and anothermaterial which can obtain light emission efficiently from the singletexcited state are mixed to be used.

As the material which can generate a singlet excited state from atriplet excited state, a thermally activated delayed fluorescentsubstance is used.

In this specification and the like, a thermally activated delayedfluorescent substance is a material which can generate a singlet excitedstate from a triplet excited state by reverse intersystem crossing andthermal activation. The thermally activated delayed fluorescentsubstance may include a material which can generate a singlet excitedstate by itself from a triplet excited state by reverse intersystemcrossing, for example, a material which emits TADF. Alternatively, thethermally activated delayed fluorescent substance may include acombination of two kinds of materials which form an exciplex.

It also can be said that the thermally activated delayed fluorescentsubstance is a material of which a triplet excited state is close to asinglet excited state. Specifically, a material in which the differencebetween the levels of the triplet excited state and the singlet excitedstate is 0.2 eV or less is preferably used. That is, it is preferablethat the difference between the levels of the triplet excited state andthe singlet excited state be 0.2 eV or less in a material which cangenerate a singlet excited state by itself from a triplet excited stateby reverse intersystem crossing, for example, a material which emitsTADF, or it is preferable that the difference between the levels of thetriplet excited state and the singlet excited state be 0.2 eV or less inan exciplex.

As a material which can obtain light emission efficiently from thesinglet excited state, a known fluorescent material is used. Inparticular, a material having high fluorescence quantum yield, forexample, a material whose fluorescence quantum yield is 50% or more, ispreferably used.

As described above, one embodiment of the present invention provides alight-emitting element in which a thermally activated delayedfluorescent substance is used for an energy donor and a fluorescentmaterial is used for an energy acceptor. With such a structure, bymaking the emission spectrum of the thermally activated delayedfluorescent substance overlap with an absorption band on the longestwavelength side in absorption by the fluorescent material in a singletexcited state, energy of a singlet excited state of the thermallyactivated delayed fluorescent substance can be transferred to thesinglet excited state of the fluorescent material. Alternatively, it isalso possible that a singlet excited state of the thermally activateddelayed fluorescent substance is generated from part of the energy of atriplet excited state of the thermally activated delayed fluorescentsubstance, and is transferred to the singlet excited state of thefluorescent material.

For example, in the case of a structure using a material which emitsTADF for an energy acceptor, a material which emits TADF and has highfluorescence quantum yield is needed in order to increase luminousefficiency. However, with the above-described structure in which athermally activated delayed fluorescent substance is used for an energydonor, a material having high fluorescence quantum yield can be selectedfor an energy acceptor with or without TADF.

Thus, the singlet excited state of the thermally activated delayedfluorescent substance and the singlet excited state of the thermallyactivated delayed fluorescent substance which is generated from part ofthe energy of the triplet excited state of the thermally activateddelayed fluorescent substance can be converted into light emission moreefficiently through the singlet excited state of the fluorescentmaterial. Accordingly, a light-emitting element having high luminousefficiency can be formed.

One embodiment of the present invention is a light-emitting elementwhich includes a pair of electrodes and an EL layer sandwiched betweenthe pair of electrodes. The EL layer includes at least a light-emittinglayer. The light-emitting layer includes at least a thermally activateddelayed fluorescent substance and a fluorescent material.

In the above light-emitting element, it is preferable that the thermallyactivated delayed fluorescent substance include a first organic compoundand a second organic compound which form an exciplex.

In the above light-emitting element, it is preferable that lightemission of the thermally activated delayed fluorescent substance beoverlapped with an absorption band on the lowest energy side of thefluorescent material.

In the above light-emitting element, it is preferable that thedifference in equivalent energy value between the peak wavelength in theabsorption band on the lowest energy side of the fluorescent materialand the peak wavelength of light emission of the thermally activateddelayed fluorescent substance be 0.2 eV or less.

In the above light-emitting element, it is preferable that thedifference between the peak wavelength of light emission of thethermally activated delayed fluorescent substance and the peakwavelength of light emission of the fluorescent material be 30 nm orless.

In the above light-emitting element, it is preferable that one of thefirst organic compound and the second organic compound be a materialhaving an electron-transport property and the other be a material havinga hole-transport property.

In the above light-emitting element, it is preferable that one of thefirst organic compound and the second organic compound be a π-electrondeficient heteroaromatic compound and the other be a π-electron richheteroaromatic compound or an aromatic amine compound.

According to one embodiment of the present invention, in alight-emitting element using a fluorescent material as a light-emittingsubstance, higher luminous efficiency can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are conceptual diagrams of light-emitting elements.

FIG. 2 illustrates energy transfer in a light-emitting layer.

FIGS. 3A and 3B are schematic diagrams of a lighting device.

FIGS. 4A and 4B are schematic diagrams of a passive matrixlight-emitting device.

FIGS. 5A and 5B are schematic diagrams of active matrix light-emittingdevices.

FIG. 6 is a schematic diagram of an active matrix light-emitting device.

FIGS. 7A to 7E illustrate electronic devices.

FIGS. 8A and 8B illustrate lighting devices.

FIGS. 9A to 9C illustrate in-vehicle display devices and electronicdevices.

FIGS. 10A and 10B show emission wavelengths of exciplexes.

FIG. 11 shows a structure of a light-emitting element 1 and a comparisonlight-emitting element 1 of Example 1.

FIG. 12 shows voltage-luminance characteristics of the light-emittingelement 1 and the comparison light-emitting element 1 of Example 1.

FIG. 13 shows luminance-current efficiency characteristics of thelight-emitting element 1 and the comparison light-emitting element 1 ofExample 1.

FIG. 14 shows luminance-power efficiency characteristics of thelight-emitting element 1 and the comparison light-emitting element 1 ofExample 1.

FIG. 15 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 1 and the comparison light-emitting element 1of Example 1.

FIG. 16 shows voltage-luminance characteristics of a light-emittingelement 2 and a comparison light-emitting element 2 of Example 2.

FIG. 17 shows luminance-current efficiency characteristics of thelight-emitting element 2 and the comparison light-emitting element 2 ofExample 2.

FIG. 18 shows voltage-current characteristics of the light-emittingelement 2 and the comparison light-emitting element 2 of Example 2.

FIG. 19 shows luminance-power efficiency characteristics of thelight-emitting element 2 and the comparison light-emitting element 2 ofExample 2.

FIG. 20 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 2 and the comparison light-emitting element 2of Example 2.

FIG. 21 shows emission spectra of the light-emitting element 2 and thecomparison light-emitting element 2 of Example 2.

FIG. 22 shows results obtained by reliability tests of thelight-emitting element 2 and the comparison light-emitting element 2 ofExample 2.

FIG. 23 shows voltage-luminance characteristics of a light-emittingelement 3 and a comparison light-emitting element 3 of Example 3.

FIG. 24 shows luminance-current efficiency characteristics of thelight-emitting element 3 and the comparison light-emitting element 3 ofExample 3.

FIG. 25 shows voltage-current characteristics of the light-emittingelement 3 and the comparison light-emitting element 3 of Example 3.

FIG. 26 shows luminance-power efficiency characteristics of thelight-emitting element 3 and the comparison light-emitting element 3 ofExample 3.

FIG. 27 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 3 and the comparison light-emitting element 3of Example 3.

FIG. 28 shows an emission spectrum of the light-emitting element 3 andthe comparison light-emitting element 3 of Example 3.

FIG. 29 shows results obtained by reliability tests of thelight-emitting element 3 and the comparison light-emitting element 3 ofExample 3.

FIG. 30 shows voltage-luminance characteristics of a light-emittingelement 4 and a comparison light-emitting element 4 of Example 4.

FIG. 31 shows luminance-current efficiency characteristics of thelight-emitting element 4 and the comparison light-emitting element 4 ofExample 4.

FIG. 32 shows voltage-current characteristics of the light-emittingelement 4 and the comparison light-emitting element 4 of Example 4.

FIG. 33 shows luminance-power efficiency characteristics of thelight-emitting element 4 and the comparison light-emitting element 4 ofExample 4.

FIG. 34 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 4 and the comparison light-emitting element 4of Example 4.

FIG. 35 shows emission spectra of the light-emitting element 4 and thecomparison light-emitting element 4 of Example 4.

FIG. 36 shows results obtained by reliability tests of thelight-emitting element 4 and the comparison light-emitting element 4 ofExample 4.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are described below with referenceto the drawings. Note that the present invention is not limited to thefollowing description, and it is easily understood by those skilled inthe art that various changes and modifications can be made withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention should not be construed as being limited to thedescription in the following embodiments.

Embodiment 1

In a light-emitting element in which a thermally activated delayedfluorescent substance and a fluorescent material are mixed to be used,light emission occurs through the following energetic process.

(1) where an electron and a hole are recombined in a fluorescentmaterial, and the fluorescent material is excited (direct recombinationprocess)

(1-1) where the fluorescent material emits fluorescence when the excitedstate of the fluorescent material is a singlet excited state

(1-2) where thermal deactivation occurs when the excited state of thefluorescent material is a triplet excited state

In the direct recombination process in (1), when the fluorescencequantum efficiency is high, high luminous efficiency can be obtained.The level of the singlet excited state of the thermally activateddelayed fluorescent substance is preferably higher than the level of thesinglet excited state of the fluorescent material.

(2) where an electron and a hole are recombined in a thermally activateddelayed fluorescent substance and the thermally activated delayedfluorescent substance is put in an excited state (energy transferprocess)

(2-1) when the excited state of the thermally activated delayedfluorescent substance is a singlet excited state

In the case where the level of the singlet excited state of thethermally activated delayed fluorescent substance is higher than thelevel of the singlet excited state of the fluorescent material,excitation energy is transferred from the thermally activated delayedfluorescent substance to the fluorescent material, and thus, thefluorescent material is put in a singlet excited state. The fluorescentmaterial in the singlet excited state emits fluorescence. Note thatsince direct transition of the fluorescent material from a singletground state to a triplet excited state is forbidden, energy transferfrom the level of the singlet excited state of the thermally activateddelayed fluorescent substance to the level of the triplet excited stateof the fluorescent material is unlikely to be a main energy transferprocess; therefore, a description thereof is omitted here. In otherwords, energy transfer from the thermally activated delayed fluorescentsubstance in the singlet excited state (¹H*) to the fluorescent materialin the singlet excited state (¹G*) is important as represented byFormula (2-1) below (where ¹G represents the singlet ground state of thefluorescent material and ¹H represents the singlet ground state of thethermally activated delayed fluorescent substance).

¹ H*+ ¹ G→ ¹ H+ ¹ G*  (2-1)

(2-2) when the excited state of the thermally activated delayedfluorescent substance is a triplet excited state

In the case where the level of the singlet excited state of thethermally activated delayed fluorescent substance is higher than thelevel of the singlet excited state of the fluorescent material, light isemitted through the following steps. First, excitation energy istransferred from the level of the triplet excited state of the thermallyactivated delayed fluorescent substance to the level of the singletexcited state of the thermally activated delayed fluorescent substanceby reverse intersystem crossing. Then, the excitation energy istransferred from the level of the singlet excited state of the thermallyactivated delayed fluorescent substance to the level of the singletexcited state of the fluorescent material, so that the fluorescentmaterial is brought into the singlet excited state. The fluorescentmaterial in the singlet excited state emits fluorescence.

In other words, as in Formula (2-2) below, the singlet excited state(¹H*) of the thermally activated delayed fluorescent substance isgenerated from the triplet excited state (³H*) of the thermallyactivated delayed fluorescent substance by reverse intersystem crossing,and then energy is transferred to the singlet excited state (¹G*) of thefluorescent material.

³ H*+ ¹ G→(reverse intersystem crossing)→¹ H*+ ¹ G→ ¹ H+ ¹ G*  (2-2)

When all the energy transfer processes described above in (2) occurefficiently, both the triplet excitation energy and the singletexcitation energy of the thermally activated delayed fluorescentsubstance are efficiently converted into the singlet excited state (¹G*)of the fluorescent material. Thus, high-efficiency light emission ispossible. In contrast, before the excitation energy of the thermallyactivated delayed fluorescent substance is transferred to thefluorescent material, when the thermally activated delayed fluorescentsubstance itself is deactivated by emitting the excitation energy aslight or heat, the luminous efficiency is decreased.

Next, factors controlling the above-described processes ofintermolecular energy transfer between the thermally activated delayedfluorescent substance and the fluorescent material are described. Asmechanisms of the intermolecular energy transfer, two mechanisms, i.e.,Förster mechanism and Dexter mechanism, have been proposed.

In Förster mechanism (dipole-dipole interaction), energy transfer doesnot require direct contact between molecules and energy is transferredthrough a resonant phenomenon of dipolar oscillation between a thermallyactivated delayed fluorescent substance and a fluorescent material. Bythe resonant phenomenon of dipolar oscillation, the thermally activateddelayed fluorescent substance provides energy to the fluorescentmaterial, and thus, the thermally activated delayed fluorescentsubstance is put in a ground state and the fluorescent material is putin an excited state. Note that the rate constant k_(h*→g) of Förstermechanism is expressed by Formula (1).

$\begin{matrix}\left\lbrack {{Formula}1} \right\rbrack &  \\{k_{h^{*}\rightarrow g} = {\frac{9000c^{4}K^{2}\phi\ln 10}{128\pi^{5}n^{4}N\tau R^{6}}{\int{\frac{{f_{h}^{\prime}(v)}{\varepsilon_{g}(v)}}{v^{4}}{dv}}}}} & (1)\end{matrix}$

In Formula (1), ν denotes a frequency, f′_(h)(ν) denotes a normalizedemission spectrum of a thermally activated delayed fluorescent substance(a fluorescent spectrum in energy transfer from a singlet excited state,and a phosphorescent spectrum in energy transfer from a triplet excitedstate), ε_(g)(ν) denotes a molar absorption coefficient of a fluorescentmaterial, N denotes Avogadro's number, n denotes a refractive index of amedium, R denotes an intermolecular distance between the thermallyactivated delayed fluorescent substance and the fluorescent material, τdenotes a measured lifetime of an excited state (fluorescence lifetimeor phosphorescence lifetime), ϕ denotes a luminescence quantum yield (afluorescence quantum yield in energy transfer from a singlet excitedstate, and a phosphorescence quantum yield in energy transfer from atriplet excited state), and K² denotes a coefficient (0 to 4) oforientation of a transition dipole moment between the thermallyactivated delayed fluorescent substance and the fluorescent material.Note that K²=2/3 in random orientation.

In Dexter mechanism (electron exchange interaction), a thermallyactivated delayed fluorescent substance and a fluorescent material areclose to a contact effective range where their orbitals overlap, and thethermally activated delayed fluorescent substance in an excited stateand the fluorescent material in a ground state exchange their electrons,which leads to energy transfer. Note that the rate constant k_(h*→g) ofDexter mechanism is expressed by Formula (2).

$\begin{matrix}\left\lbrack {{Formula}2} \right\rbrack &  \\{k_{h^{*}\rightarrow g} = {\left( \frac{2\pi}{h} \right)K^{2}{\exp\left( {- \frac{2R}{L}} \right)}{\int{{f_{h}^{\prime}(v)}{\varepsilon_{g}^{\prime}(v)}{dv}}}}} & (2)\end{matrix}$

In Formula (2), h denotes a Planck constant, K denotes a constant havingan energy dimension, ν denotes a frequency, f′_(h)(ν) denotes anormalized emission spectrum of a thermally activated delayedfluorescent substance (a fluorescent spectrum in energy transfer from asinglet excited state, and a phosphorescent spectrum in energy transferfrom a triplet excited state), ε′_(g)(ν) denotes a normalized absorptionspectrum of a fluorescent material, L denotes an effective molecularradius, and R denotes an intermolecular distance between the thermallyactivated delayed fluorescent substance and the fluorescent material.

Here, the energy transfer efficiency Φ_(ET) from the thermally activateddelayed fluorescent substance to the fluorescent material is thought tobe expressed by Formula (3). In the formula, k_(r) denotes a rateconstant of a light-emission process (fluorescence in energy transferfrom a singlet excited state, and phosphorescence in energy transferfrom a triplet excited state) of a thermally activated delayedfluorescent substance, k_(n) denotes a rate constant of anon-light-emission process (thermal deactivation or intersystemcrossing) of a thermally activated delayed fluorescent substance, and rdenotes a measured lifetime of an excited state of a thermally activateddelayed fluorescent substance.

$\begin{matrix}\left\lbrack {{Formula}3} \right\rbrack &  \\{\Phi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & (3)\end{matrix}$

According to Formula (3), it is found that the energy transferefficiency Φ_(ET) can be increased by increasing the rate constantk_(h*→g) of energy transfer so that another competing rate constantk_(r)+k_(n) (=1/τ) becomes relatively small.

In both the energy transfer processes of (2-1) and (2-2), since energyis transferred from the singlet excited state (¹H*) of the thermallyactivated delayed fluorescent substance to the fluorescent material,energy transfers by both Förster mechanism (Formula (1)) and Dextermechanism (Formula (2)) are possible.

First, an energy transfer by Förster mechanism is considered. When τ iseliminated from Formula (1) and Formula (3), it can be said that theenergy transfer efficiency Φ_(ET) is higher when the quantum yield ϕ(here, a fluorescence quantum efficiency because energy transfer from asinglet excited state is discussed) is higher. However, in practice, amore important factor is that the emission spectrum of the thermallyactivated delayed fluorescent substance (here, a fluorescent spectrumbecause energy transfer from a singlet excited state is discussed)largely overlaps with the absorption spectrum of the fluorescentmaterial (absorption corresponding to the transition from the singletground state to the singlet excited state) (note that it is preferablethat the molar absorption coefficient of the fluorescent material bealso high). This means that the fluorescent spectrum of the thermallyactivated delayed fluorescent substance overlaps with the absorptionband on the longest wavelength side of the fluorescent material.

Next, an energy transfer by Dexter mechanism is considered. According toFormula (2), in order to increase the rate constant k_(h*→g), it ispreferable that an emission spectrum of a thermally activated delayedfluorescent substance (here, a fluorescent spectrum because energytransfer from a singlet excited state is discussed) largely overlap withan absorption spectrum of a fluorescent material (absorptioncorresponding to transition from a singlet ground state to a singletexcited state).

The above description suggests that in both the energy transferprocesses of (2-1) and (2-2), the energy transfer efficiency can beoptimized by making the emission spectrum of the thermally activateddelayed fluorescent substance overlap with the absorption band on thelongest wavelength side of the fluorescent material.

In order to increase the luminous efficiency of the light-emittingelement, it is important that the thermally activated delayedfluorescent substance generates a singlet excited state from a tripletexcited state and the fluorescent material has high fluorescence quantumyield.

However, it is very difficult to form a material which can generate asinglet excited state from a triplet excited state and has highfluorescence quantum yield.

It is preferable that the ratio of the energy transfer process of (2) behigh and the ratio of the direct recombination process of (1) be lowbecause the thermal deactivation process of (1-2) can be reduced. Thus,the concentration of the fluorescent material is preferably 5 wt % orlower, more preferably 1 wt % or lower.

Therefore, one embodiment of the present invention provides an effectivetechnique which can overcome problems of the energy transfer efficiencyfrom the thermally activated delayed fluorescent substance in thetriplet excited state to the fluorescent material and the fluorescencequantum efficiency of the singlet excited state of the fluorescentmaterial in the case where the fluorescent material is used as alight-emitting substance. Specific embodiments thereof are describedbelow.

One embodiment of the present invention provides a light-emittingelement in which a thermally activated delayed fluorescent substance isused as an energy donor capable of efficiently transferring energy to afluorescent material. The thermally activated delayed fluorescentsubstance has a feature that its singlet and triplet excited states areclose to each other. Thus, in the thermally activated delayedfluorescent substance, a triplet excited state is easily transferred toa singlet excited state. By making the emission spectrum of thethermally activated delayed fluorescent substance overlap with anabsorption band on the longest wavelength side in absorption by thefluorescent material, i.e., an energy acceptor, in a singlet excitedstate (an absorption corresponding to the transition from the singletground state to the singlet excited state), it becomes possible toimprove the energy transfer efficiency from the triplet excited stateand the singlet excited state of the thermally activated delayedfluorescent substance to the singlet excited state of the fluorescentmaterial.

In the case where a light-emitting substance includes a material forgenerating a singlet excited state from a triplet excited state andanother material for obtaining light emission efficiently from thesinglet excited state, a material having high fluorescence quantumyield, for example, a material whose fluorescence quantum yield is 50%or more, can be selected as the light-emitting substance with or withoutthermally activated delay.

Thus, the energy of the singlet excited state and the triplet excitedstate of the thermally activated delayed fluorescent substance can beconverted into light emission more efficiently through the singletexcited state of the fluorescent material. Accordingly, a light-emittingelement having high luminous efficiency can be formed.

In a light-emitting element having the above structure, energy transferoccurs efficiently as illustrated in FIG. 2. FIG. 2 shows that alight-emitting layer 113 is provided between an electrode 101 and anelectrode 102. There may be a given layer between each electrode and thelight-emitting layer 113. Energy is transferred from a singlet excitedstate S_(D) of a thermally activated delayed fluorescent substance 113Dto a singlet excited state S_(A) of a light-emitting substance 113A.Further, a triplet excited state T_(D) of the thermally activateddelayed fluorescent substance 113D is changed to the singlet excitedstate S_(D) of the thermally activated delayed fluorescent substance113D by reverse intersystem crossing, and then energy is transferred tothe singlet excited state S_(A) of the light-emitting substance 113A.Then, the singlet excited state S_(A) of the light-emitting substance113A emits light. As described above, in the light-emitting element ofone embodiment of the present invention, energy transfer and lightemission are performed well by including a material for generating asinglet excited state from a triplet excited state and another materialfor obtaining light emission efficiently from the singlet excited state;thus, the light-emitting element can have high luminous efficiency.

FIGS. 1A to 1C are schematic diagrams of the light-emitting element ofthis embodiment. FIG. 1A is a diagram of the light-emitting element, andFIGS. 1B and 1C are enlarged diagrams of only the light-emitting layer113.

The light-emitting element includes an EL layer 103 between a pair ofelectrodes, the first electrode 101 and the second electrode 102, andthe EL layer 103 contains an organic compound as a light-emittingsubstance. In addition, the EL layer includes the light-emitting layer113, and the light-emitting substance is contained at least in thelight-emitting layer 113. There is no limitation on layers other thanthe light-emitting layer 113, and any layer may be used as the otherlayers. A typical stacked-layer structure includes a hole-injectionlayer 111, a hole-transport layer 112, an electron-transport layer 114,an electron-injection layer 115, and the like. Besides, acarrier-blocking layer or the like may be provided, or a plurality oflight-emitting layers may be provided.

The light-emitting layer 113 includes the thermally activated delayedfluorescent substance 113D and the light-emitting substance 113A. Asillustrated in FIG. 1B, the thermally activated delayed fluorescentsubstance 113D may include a material which can generate a singletexcited state by itself from a triplet excited state by reverseintersystem crossing. The thermally activated delayed fluorescentsubstance 113D may include a plurality of materials. As illustrated inFIG. 1C, it is particularly preferable that the thermally activateddelayed fluorescent substance 113D include two kinds of materials, whichare a first organic compound 113D1 and a second organic compound 113D2which form an exciplex. An exciplex has a small difference between thelevel of the singlet excited state and the level of the triplet excitedstate, and thus energy is easily transferred from the level of thetriplet excited state to the level of the singlet excited state in theexciplex. Thus, the thermally activated delayed fluorescent substanceformed using the combination of the first organic compound and thesecond organic compound which form an exciplex is suitable for thethermally activated delayed fluorescent substance of one embodiment ofthe present invention. Further, in terms of luminous efficiency andreliability, it is preferable to use a material having a hole-transportproperty as one of the first organic compound and the second organiccompound and to use a material having an electron-transport property asthe other because the carrier balance between holes and electrons in thelight-emitting layer can be easily optimized by adjustment of themixture ratio of the first organic compound and the second organiccompound. Note that this does not exclude the possibility that thelight-emitting layer 113 in the light-emitting element of thisembodiment contains another substance.

In the thermally activated delayed fluorescent substance, the singletexcited state is close to the triplet excited state; in particular, theenergy difference between the singlet excited state and the tripletexcited state is preferably larger than or equal to 0 eV and smallerthan or equal to 0.2 eV.

The thermally activated delayed fluorescent substance and thefluorescent material are preferably combined so that light emission ofthe thermally activated delayed fluorescent substance is overlapped withan absorption band on the longest wavelength side of the light-emittingsubstance 113A as described above. Accordingly, energy is efficientlytransferred from the singlet excited state of the thermally activateddelayed fluorescent substance to the singlet excited state of thefluorescent material.

As examples of a fluorescent material which can be used for thelight-emitting substance 113A, the following can be given. Examples ofthe fluorescent substance are5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation:PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine(abbreviation: PAPP2BPy),N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N-diphenyl-pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″N″,N′″,N′″-octaphenyldibenzo[g,p]chhrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone(abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H1-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB),2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM),2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[Q]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM), and the like.

The concentration of the fluorescent material in the light-emittinglayer 113 is preferably 5 wt % or lower, more preferably 1 wt % orlower. With such a concentration, the ratio of the energy transferprocess of (2) can be increased and the ratio of the directrecombination process of (1) can be decreased, so that the thermaldeactivation process of (1-2) can be reduced.

In the case where the thermally activated delayed fluorescent substanceis formed using one kind of material, the following can be used, forexample.

First, a fullerene, a derivative thereof, an acridine derivative such asproflavine, and eosin can be given. Further, a metal-containingporphyrin, such as a porphyrin containing magnesium (Mg), zinc (Zn),cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd)can be given. Examples of the metal-containing porphyrin include aprotoporphyrin-tin fluoride complex (SnF₂(Proto IX)), amesoporphyrin-tin fluoride complex (SnF₂(Meso IX)), ahematoporphyrin-tin fluoride complex (SnF₂(Hemato IX)), a coproporphyrintetramethyl ester-tin fluoride complex (SnF₂(Copro III-4Me)), anoctaethylporphyrin-tin fluoride complex (SnF₂(OEP)), anetioporphyrin-tin fluoride complex (SnF₂(Etio I)), and anoctaethylporphyrin-platinum chloride complex (PtCl₂(OEP)), which areshown in the following structural formulae.

Alternatively, a heterocyclic compound having a π-electron richheteroaromatic ring and a π-electron deficient heteroaromatic ring, suchas2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]charbazol-11-yl)-1,3,5-triazine(PIC-TRZ) shown in the following structural formula, can be used as thethermally activated delayed fluorescent substance, which is formed usingone kind of material. The heterocyclic compound is preferably usedbecause of the π-electron rich heteroaromatic ring and the π-electrondeficient heteroaromatic ring, for which the electron-transport propertyand the hole-transport property are high. Note that a substance in whichthe π-electron rich heteroaromatic ring is directly bonded to theπ-electron deficient heteroaromatic ring is particularly preferably usedbecause the donor property of the π-electron rich heteroaromatic ringand the acceptor property of the π-electron deficient heteroaromaticring are both increased and the difference between the level of thesinglet excited state and the level of the triplet excited state becomessmall.

As the thermally activated delayed fluorescent substance, two kinds oforganic compounds, which are the first organic compound and the secondorganic compound which form an exciplex, can be used. In this case, aknown carrier-transport material can be used as appropriate. In order toform an exciplex efficiently, it is particular preferable to combine acompound which easily accepts electrons (a compound having anelectron-transport property) and a compound which easily accepts holes(a compound having a hole-transport property).

This is because the carrier balance between holes and electrons in thelight-emitting layer can be easily optimized by the use of thecombination of a material having an electron-transport property and amaterial having a hole-transport property as the thermally activateddelayed fluorescent substance and by adjustment of the mixture ratio ofthe material having an electron-transport property and the materialhaving a hole-transport property. The optimization of the carrierbalance between holes and electrons in the light-emitting layer canprevent a region in which electrons and holes are recombined fromexisting on one side in the light-emitting layer. By preventing theregion in which electrons and holes are recombined from existing to oneside, the reliability of the light-emitting element can be improved.

As the compound which easily accepts electrons (the material having anelectron-transport property), a π-electron deficient heteroaromaticcompound, a metal complex, or the like can be used. Specific examplesinclude a metal complex such asbis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); aheterocyclic compound having a polyazole skeleton such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), or2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); a heterocyclic compound having a diazineskeleton such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), or4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and a heterocyclic compound having a pyridine skeletonsuch as 3,5-bis[3-(91H-carbazol-9-yl)phenyl]pyridine (abbreviation:35DCzPPy) or 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation:TmPyPB). Among the above materials, a heterocyclic compound having adiazine skeleton and a heterocyclic compound having a pyridine skeletonhave high reliability and are thus preferable. Specifically, aheterocyclic compound having a diazine (pyrimidine or pyrazine) skeletonhas a high electron-transport property to contribute to a reduction indrive voltage.

As the compound which easily accepts holes (material having ahole-transport property), a π-electron rich heteroaromatic compound, anaromatic amine compound, or the like can be favorably used. Specificexamples include a compound having an aromatic amine skeleton such as2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-fluoren-2-amine(abbreviation: PCBAF), orN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); a compound having a carbazole skeleton such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or9-phenyl-9H-3-(9-phenyl-9H-carbazol-3-yl)carbazole (abbreviation: PCCP);a compound having a thiophene skeleton such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), or4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and a compound having a furan skeleton suchas 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation:DBF3P-II) or4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, a compoundhaving an aromatic amine skeleton and a compound having a carbazoleskeleton are preferable because these compounds are highly reliable andhave high hole-transport properties to contribute to a reduction indrive voltage.

The first organic compound and the second organic compound are notlimited to these examples, as long as they can transport carriers, thecombination can form an exciplex, and light emission of the exciplexoverlaps with an absorption band on the longest wavelength side in anabsorption spectrum of a light-emitting substance (an absorptioncorresponding to the transition of the light-emitting substance from thesinglet ground state to the singlet excited state), and other knownmaterials may be used.

Note that in the case where a material having an electron-transportproperty and a material having a hole-transport property are used as thefirst organic compound and the second organic compound, carrier balancecan be controlled by the mixture ratio of the compounds. Specifically,the ratio of the first organic compound to the second organic compoundis preferably 1:9 to 9:1.

Here, compounds which form an exciplex (the first organic compound 113D1and the second organic compound 113D2) and the exciplex are described ina little more detail.

FIGS. 10A and 10B show emission spectra of four kinds of organiccompounds and emission spectra of exciplexes formed using the organiccompounds. Note that in the figures, a compound 1 is2-[4-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: DBTBIm-II); a compound 2 is2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II); a compound 3 is4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA); a compound 4 is2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene(abbreviation: DPA2SF). An exciplex 1 is an exciplex of the compound 1and the compound 3. An exciplex 2 is an exciplex of the compound 2 andthe compound 3. An exciplex 3 is an exciplex of the compound 2 and4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB). Anexciplex 4 is an exciplex of the compound 2 and the compound 4.

Structural formulae of the compounds are shown below.

FIG. 10A shows emission spectra of the exciplexes 1 and 2 and thecompounds 1 to 3. The spectrum of the exciplex 1 is the result ofmeasuring light emission of a material based on the compound 1 to whicha slight amount of compound 3 is added, and the spectrum of the exciplex2 is the result of measuring light emission of a material based on thecompound 2 to which a slight amount of compound 3 is added. That is, ina sample used for measurement of the exciplex 1, one of the compounds 1and 3 corresponds to the first organic compound 113D1, and the othercorresponds to the second organic compound 113D2. In a sample used formeasurement of the exciplex 2, one of the compounds 2 and 3 correspondsto the first organic compound 113D1, and the other corresponds to thesecond organic compound 113D2.

As can be seen from FIG. 10A, there is a difference of 100 nm or morebetween light emission of the exciplex 1 and light emission of theexciplex 2 even though both materials contain the compound 3 as aslight-amount component. This means that the emission wavelength of anexciplex can be easily adjusted by changing a base substance.

The peak wavelength of the emission spectrum of the exciplex 1 isapproximately 520 nm, and thus the thermally activated delayedfluorescent substance containing the compound 1 and the compound 3 canbe preferably used together with a material which emits blue-green tored fluorescence.

The peak wavelength of the emission spectrum of the exciplex 2 isapproximately 610 nm, and thus the thermally activated delayedfluorescent substance containing the compound 2 and the compound 3 canbe preferably used together with a material which emits redfluorescence.

FIG. 10B shows emission spectra of the exciplexes 3 and 4 and thecompounds 2 and 4. The spectrum of the exciplex 3 is the result ofmeasuring light emission of a material based on the compound 2 to whicha slight amount of NPB is added, and the spectrum of the exciplex 4 isthe result of measuring light emission of a material based on thecompound 2 to which a slight amount of compound 4 is added. That is, ina sample used for measurement of the exciplex 3, one of the compound 2and NPB corresponds to the first organic compound 113D1, and the othercorresponds to the second organic compound 113D2. In a sample used formeasurement of the exciplex 4, one of the compounds 2 and 4 correspondsto the first organic compound 113D1, and the other corresponds to thesecond organic compound 113D2.

As can be seen from FIG. 10B, there is a difference of about 100 nmbetween light emission of the exciplex 3 and light emission of theexciplex 4 even though both materials contain the same base material.This means that the emission wavelength of an exciplex can be easilyadjusted by changing a substance that is a slight-amount component.

The peak wavelength of the emission spectrum of the exciplex 3 isapproximately 520 nm, and thus the thermally activated delayedfluorescent substance containing the compound 2 and NPB can bepreferably used together with a material which emits blue-green to redfluorescence.

The peak wavelength of the emission spectrum of the exciplex 4 isapproximately 580 nm, and thus the thermally activated delayedfluorescent substance containing the compounds 2 and 4 can be preferablyused together with a material which emits orange to red fluorescence.

The light-emitting element having the above structure has high energytransfer efficiency to the fluorescent material and has high luminousefficiency.

In the case where the two kinds of organic compounds which form anexciplex are used as the thermally activated delayed fluorescentsubstance, the driving voltage of the light-emitting element can belowered, which is also preferable. By lowering the driving voltage, alight-emitting element with low power consumption can be formed. Thereason why the driving voltage of the light-emitting element can belowered by the use of the exciplex is described below.

In the case where the organic compounds which form an exciplex are usedas the thermally activated delayed fluorescent substance, the thresholdvalue of the voltage at which the exciplex is formed by carrierrecombination (or a singlet exciton) is determined depending on theenergy of the peak of the emission spectrum of the exciplex. When theemission spectrum of the exciplex peaks at 620 nm (2.0 eV), for example,the threshold value of voltage needed when the exciplex is formed withelectric energy is also approximately 2.0 V.

Here, when the energy of the peak of the emission spectrum of theexciplex is too high (i.e., when the wavelength is too short), thethreshold value of the voltage with which an exciplex is formed alsoincreases. That case is not preferred because higher voltage is neededto make the fluorescent material emit light by energy transfer from theexciplex to the fluorescent material, and thus extra energy is consumed.From this point of view, it is preferable that energy of the peak of theemission spectrum of the exciplex be lower (the wavelength be longer)because the threshold value of the voltage is lowered.

Thus, the peak wavelength of the emission spectrum of the exciplex ismade to be longer than or equal to the peak wavelength of the absorptionband on the longest wavelength side in the absorption spectrum of thefluorescent material, whereby a light-emitting element with low drivingvoltage can be obtained. Even in this case, energy can be transferred byutilizing an overlap of the emission spectrum of the exciplex with theabsorption band on the longest wavelength side in the absorptionspectrum of the fluorescent material; thus, high luminous efficiency canbe obtained. As described above, high luminous efficiency (externalquantum efficiency) is obtained with the drive voltage reduced, wherebyhigh power efficiency can be achieved.

In the light-emitting element, the threshold voltage at which anexciplex is formed due to the carrier recombination is lower than thethreshold voltage at which the fluorescent material starts to emit lightdue to the carrier recombination. In other words, even when voltage thatis lower than the threshold voltage with which the fluorescent materialstarts to emit light is applied to the light-emitting element, carrierrecombination occurs and an exciplex is formed; thus, recombinationcurrent starts to flow through the light-emitting element. Therefore, alight-emitting element with lower drive voltage (with more favorablevoltage-current characteristics) can be provided.

Accordingly, at the time when the voltage reaches the threshold valuewith which the fluorescent material starts to emit light, a sufficientnumber of carriers exist in the light-emitting layer and carrierrecombination which can contribute to light emission of the fluorescentmaterial smoothly occurs many times. Therefore, luminance becomesremarkably high at a voltage close to the threshold voltage (lightemission start voltage) of the fluorescent material. In other words, acurve representing the voltage-luminance characteristics can be steep ina rising portion near the emission start voltage; thus, drive voltageneeded to obtain desired luminance can be low. Further, to obtainpractical luminance, driving is performed with voltage higher than orequal to the threshold voltage (light emission start voltage) of thefluorescent material, in which case emitted light originates mostly fromthe fluorescent material and the light-emitting element is thus allowedto have high current efficiency.

The effect of the reduction in voltage is seen notably when the peak ofthe emission spectrum of the exciplex is located in a region rangingfrom the peak of the emission spectrum of the fluorescent material to awavelength 30 nm longer than the peak of the emission spectrum of thefluorescent material or when the difference in equivalent energy valuebetween peak wavelength of the emission spectrum of the exciplex and thepeak wavelength of the emission spectrum of the fluorescent material issmaller than or equal to +0.2 eV. In the case of a region when the peakof the emission spectrum of the exciplex is located in a region rangingfrom the peak of the emission spectrum of the fluorescent material to awavelength 30 nm shorter than the peak of the emission spectrum of thefluorescent material or when the difference in equivalent energy valuebetween peak wavelength of the emission spectrum of the exciplex and thepeak wavelength of the emission spectrum of the fluorescent material isgreater than or equal to −0.2 eV, relatively high luminous efficiencycan be kept.

Embodiment 2

In this embodiment, a detailed example of the structure of thelight-emitting element described in Embodiment 1 is described below withreference to FIGS. 1A to 1C.

A light-emitting element in this embodiment includes, between a pair ofelectrodes, an EL layer including a plurality of layers. In thisembodiment, the light-emitting element includes the first electrode 101,the second electrode 102, and the EL layer 103 which is provided betweenthe first electrode 101 and the second electrode 102. Note that thefollowing description in this embodiment is made on the assumption thatthe first electrode 101 functions as an anode and that the secondelectrode 102 functions as a cathode. In other words, when a voltage isapplied between the first electrode 101 and the second electrode 102 sothat the potential of the first electrode 101 is higher than that of thesecond electrode 102, light emission can be obtained.

Since the first electrode 101 functions as the anode, the firstelectrode 101 is preferably formed using any of metals, alloys,electrically conductive compounds with a high work function(specifically, a work function of 4.0 eV or more), mixtures thereof, andthe like. Specifically, for example, indium oxide-tin oxide (ITO: indiumtin oxide), indium oxide-tin oxide containing silicon or silicon oxide,indium oxide-zinc oxide, indium oxide containing tungsten oxide and zincoxide (IWZO), and the like can be given. Films of these electricallyconductive metal oxides are usually formed by a sputtering method butmay be formed by application of a sol-gel method or the like. In anexample of the formation method, indium oxide-zinc oxide is deposited bya sputtering method using a target obtained by adding 1 wt % to 20 wt %of zinc oxide to indium oxide. Further, a film of indium oxidecontaining tungsten oxide and zinc oxide (IWZO) can be formed by asputtering method using a target in which tungsten oxide and zinc oxideare added to indium oxide at 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %,respectively. Besides, gold (Au), platinum (Pt), nickel (Ni), tungsten(W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper(Cu), palladium (Pd), nitrides of metal materials (e.g., titaniumnitride), and the like can be given. Graphene can also be used. Notethat when a composite material described later is used for a layer whichis in contact with the first electrode 101 in the EL layer 103, anelectrode material can be selected regardless of its work function.

There is no particular limitation on the stacked-layer structure of theEL layer 103 as long as the light-emitting layer 113 has the structuredescribed in Embodiment 1. For example, the EL layer 103 can be formedby combining a hole-injection layer, a hole-transport layer, thelight-emitting layer, an electron-transport layer, an electron-injectionlayer, a carrier-blocking layer, an intermediate layer, and the like asappropriate. In this embodiment, the EL layer 103 has a structure inwhich the hole-injection layer 111, the hole-transport layer 112, thelight-emitting layer 113, the electron-transport layer 114, and theelectron-injection layer 115 are stacked in this order over the firstelectrode 101. Specific examples of materials used for each layer aregiven below.

The hole-injection layer 111 is a layer containing a material having ahigh hole-injection property. Molybdenum oxide, vanadium oxide,ruthenium oxide, tungsten oxide, manganese oxide, or the like can beused. Alternatively, the hole-injection layer 111 can be formed using aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper phthalocyanine (abbreviation: CuPc), an aromatic aminecompound such as4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB) orN,N-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), a high molecular compound such aspoly(ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS), orthe like.

Alternatively, a composite material in which a material having ahole-transport property contains a substance having an acceptor propertycan be used for the hole-injection layer 111. Note that the use of sucha substance having a hole-transport property which contains a substancehaving an acceptor property enables selection of a material used to forman electrode regardless of its work function. In other words, besides amaterial having a high work function, a material having a low workfunction can also be used for the first electrode 101. As the substancehaving an acceptor property,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, transitionmetal oxides can be given. Oxides of the metals that belong to Groups 4to 8 of the periodic table can be given. Specifically, vanadium oxide,niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide are preferable inthat their electron-accepting property is high. Among these oxides,molybdenum oxide is particularly preferable in that it is stable in theair, has a low hygroscopic property, and is easy to handle.

As the substance having a hole-transport property which is used for thecomposite material, any of a variety of organic compounds such asaromatic amine compounds, carbazole derivatives, aromatic hydrocarbons,and high molecular compounds (e.g., oligomers, dendrimers, or polymers)can be used. Note that the organic compound used for the compositematerial is preferably an organic compound having a high hole-transportproperty. Specifically, a substance having a hole mobility of 10⁻⁶cm²/Vs or more is preferably used. Organic compounds that can be used asthe substance having a hole-transport property in the composite materialare specifically given below.

Examples of the aromatic amine compounds areN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

Specific examples of the carbazole derivatives that can be used for thecomposite material are3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

Other examples of the carbazole derivatives that can be used for thecomposite material are 4,4′-di(N-carbazolyl)biphenyl (abbreviation:CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of the aromatic hydrocarbons that can be used for the compositematerial are 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, andthe like. Besides, pentacene, coronene, or the like can also be used.The aromatic hydrocarbon which has a hole mobility of 1×10⁻⁶ cm²Ns ormore and which has 14 to 42 carbon atoms is particularly preferable.

Note that the aromatic hydrocarbons that can be used for the compositematerial may have a vinyl skeleton. Examples of the aromatic hydrocarbonhaving a vinyl group are 4,4′-bis(2,2-diphenylvinyl)biphenyl(abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA), and the like.

A high molecular compound such as poly(N-vinylcarbazole) (abbreviation:PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:poly-TPD) can also be used.

By providing the hole-injection layer 111, a high hole-injectionproperty can be achieved to allow a light-emitting element to be drivenat a low voltage.

The hole-transport layer 112 is a layer that contains a material havinga hole-transport property. Examples of the substance having ahole-transport property are aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), and the like. The substances mentioned here havehigh hole-transport properties and are mainly ones that have a holemobility of 10⁻⁶ cm²/Vs or more. An organic compound given as an exampleof the substance having a hole-transport property in the compositematerial described above can also be used for the hole-transport layer112. A high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA)can also be used. Note that the layer that contains a substance having ahole-transport property is not limited to a single layer, and may be astack of two or more layers including any of the above substances.

The light-emitting layer 113 contains at least a light-emittingsubstance and a thermally activated delayed fluorescent substance. Sincethe light-emitting layer 113 has the structure described in Embodiment1, the light-emitting element in this embodiment can have extremely highluminous efficiency. Embodiment 1 can be referred to for the maincomponents of the light-emitting layer 113.

The light-emitting layer 113 having the above-described structure can bedeposited by co-evaporation by a vacuum evaporation method, or an inkjetmethod, a spin coating method, a dip coating method, or the like using amixed solution.

The electron-transport layer 114 is a layer containing a material havingan electron-transport property. For example, a layer containing a metalcomplex having a quinoline skeleton or a benzoquinoline skeleton, suchas tris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAIq), or the like can be used. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂), orthe like can be used. Besides the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances mentioned here have high electron-transport properties andare mainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or more.Note that any of the above-described thermally activated delayedfluorescent substances having electron-transport properties may be usedfor the electron-transport layer 114.

The electron-transport layer 114 is not limited to a single layer, andmay be a stack of two or more layers containing any of the abovesubstances.

Between the electron-transport layer and the light-emitting layer, alayer that controls transport of electrons may be provided. This is alayer formed by addition of a small amount of a substance having a highelectron-trapping property to the aforementioned material having a highelectron-transport property, and the layer is capable of adjustingcarrier balance by retarding transport of electron carriers. Such astructure is very effective in preventing a problem (such as a reductionin element lifetime) caused when electrons pass through thelight-emitting layer.

In addition, the electron-injection layer 115 may be provided in contactwith the second electrode 102 between the electron-transport layer 114and the second electrode 102. For the electron-injection layer 115, analkali metal, an alkaline earth metal, or a compound thereof, such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂), can be used. For example, a layer that is formed using amaterial having an electron-transport property and contains an alkalimetal, an alkaline earth metal, or a compound thereof can be used. Notethat a layer that is formed using a material having anelectron-transport property and contains an alkali metal or an alkalineearth metal is preferably used as the electron-injection layer 115, inwhich case electron injection from the second electrode 102 isefficiently performed.

For the second electrode 102, any of metals, alloys, electricallyconductive compounds, and mixtures thereof which have a low workfunction (specifically, a work function of 3.8 eV or less) or the likecan be used. Specific examples of such a cathode material are elementsbelonging to Groups 1 and 2 of the periodic table, such as alkali metals(e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), andstrontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metalssuch as europium (Eu) and ytterbium (Yb), alloys thereof, and the like.However, when the electron-injection layer is provided between thesecond electrode 102 and the electron-transport layer, for the secondelectrode 102, any of a variety of conductive materials such as Al, Ag,ITO, or indium oxide-tin oxide containing silicon or silicon oxide canbe used regardless of the work function. Films of these electricallyconductive materials can be formed by a sputtering method, an inkjetmethod, a spin coating method, or the like.

Any of a variety of methods can be used to form the EL layer 103regardless whether it is a dry process or a wet process. For example, avacuum evaporation method, an inkjet method, a spin coating method, orthe like may be used. Different formation methods may be used for theelectrodes or the layers.

In addition, the electrode may be formed by a wet method using a sol-gelmethod, or by a wet method using paste of a metal material.Alternatively, the electrode may be formed by a dry method such as asputtering method or a vacuum evaporation method.

In the light-emitting element having the above-described structure,current flows due to a potential difference which is generated betweenthe first electrode 101 and the second electrode 102, and holes andelectrons recombine in the light-emitting layer 113 which contains alight-emitting substance, so that light is emitted. That is, alight-emitting region is formed in the light-emitting layer 113.

Light emission is extracted out through one or both of the firstelectrode 101 and the second electrode 102. Therefore, one or both ofthe first electrode 101 and the second electrode 102 arelight-transmitting electrodes. In the case where only the firstelectrode 101 is a light-transmitting electrode, light emission isextracted through the first electrode 101. In the case where only thesecond electrode 102 is a light-transmitting electrode, light emissionis extracted through the second electrode 102. In the case where boththe first electrode 101 and the second electrode 102 arelight-transmitting electrodes, light emission is extracted through thefirst electrode 101 and the second electrode 102.

The structure of the layers provided between the first electrode 101 andthe second electrode 102 is not limited to the above-describedstructure. Preferably, a light-emitting region where holes and electronsrecombine is positioned away from the first electrode 101 and the secondelectrode 102 so that quenching due to the proximity of thelight-emitting region and a metal used for electrodes andcarrier-injection layers can be prevented.

Further, in order that transfer of energy from an exciton generated inthe light-emitting layer can be suppressed, preferably, thehole-transport layer and the electron-transport layer which are incontact with the light-emitting layer 113, particularly acarrier-transport layer in contact with a side closer to thelight-emitting region in the light-emitting layer 113, are formed usinga substance having a wider band gap than the light-emitting substance.

The light-emitting element in this embodiment is provided over asubstrate of glass, plastic, a metal, or the like. As a substrate whichtransmits light from the light-emitting element, a substrate having ahigh visible light transmitting property is used. As the way of stackinglayers over a substrate which transmits light, layers may besequentially stacked from the first electrode 101 side or sequentiallystacked from the second electrode 102 side. In a light-emitting device,although one light-emitting element may be formed over one substrate, aplurality of light-emitting elements may be formed over one substrate.With a plurality of light-emitting elements as described above formedover one substrate, a lighting device in which elements are separated ora passive-matrix light-emitting device can be manufactured. Alight-emitting element may be formed over an electrode electricallyconnected to a thin film transistor (TFT), for example, which is formedover a substrate of glass, plastic, or the like, so that an activematrix light-emitting device in which the TFT controls the drive of thelight-emitting element can be manufactured. Note that there is noparticular limitation on the structure of the TFT, which may be astaggered TFT or an inverted staggered TFT. In addition, crystallinityof a semiconductor used for the TFT is not particularly limited either;an amorphous semiconductor or a crystalline semiconductor may be used.In addition, a driver circuit formed in a TFT substrate may be formedwith an n-type TFT and a p-type TFT, or with either an n-type TFT or ap-type TFT.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, an example in which the light-emitting elementdescribed in Embodiment 1 or 2 is used for a lighting device isdescribed with reference to FIGS. 3A and 3B. FIG. 3B is a top view ofthe lighting device, and FIG. 3A is a cross-sectional view taken alonge-f in FIG. 3B.

In the lighting device in this embodiment, a first electrode 401 isformed over a substrate 400 which is a support and has alight-transmitting property. The first electrode 401 corresponds to thefirst electrode 101 in Embodiment 2.

An auxiliary electrode 402 is provided over the first electrode 401.Since this embodiment shows an example in which light emission isextracted through the first electrode 401 side, the first electrode 401is formed using a material having a light-transmitting property. Theauxiliary electrode 402 is provided in order to compensate for the lowconductivity of the material having a light-transmitting property, andhas a function of suppressing luminance unevenness in a light emissionsurface due to voltage drop caused by the high resistance of the firstelectrode 401. The auxiliary electrode 402 is formed using a materialhaving at least higher conductivity than the material of the firstelectrode 401, and is preferably formed using a material having highconductivity such as aluminum. Note that surfaces of the auxiliaryelectrode 402 other than a portion thereof in contact with the firstelectrode 401 are preferably covered with an insulating layer. This isfor suppressing light emission over the upper portion of the auxiliaryelectrode 402, which cannot be extracted, for reducing a reactivecurrent, and for suppressing a reduction in power efficiency. Note thata pad 412 for applying a voltage to a second electrode 404 may be formedat the same time as the formation of the auxiliary electrode 402.

An EL layer 403 is formed over the first electrode 401 and the auxiliaryelectrode 402. The EL layer 403 has the structure described inEmbodiment 1 or 2. Note that the EL layer 403 is preferably formed to beslightly larger than the first electrode 401 when seen from above so asto also serve as an insulating layer that suppresses a short circuitbetween the first electrode 401 and the second electrode 404.

The second electrode 404 is formed to cover the EL layer 403. The secondelectrode 404 corresponds to the second electrode 102 in Embodiment 2and has a structure similar to the second electrode 102. In thisembodiment, it is preferable that the second electrode 404 be formedusing a material having high reflectance because light emission isextracted through the first electrode 401 side. In this embodiment, thesecond electrode 404 is connected to the pad 412, whereby voltage isapplied.

As described above, the lighting device described in this embodimentincludes a light-emitting element including the first electrode 401, theEL layer 403, and the second electrode 404 (and the auxiliary electrode402). Since the light-emitting element has high luminous efficiency, thelighting device in this embodiment can be a lighting device with lowpower consumption.

The light-emitting element having the above structure is fixed to asealing substrate 407 with sealing materials 405 and 406 and sealing isperformed, whereby the lighting device is completed. It is possible touse only either the sealing material 405 or the sealing material 406. Inaddition, the inner sealing material 406 can be mixed with a desiccant,whereby moisture is adsorbed and the reliability is increased.

When extended to the outside of the sealing materials 405 and 406, thepad 412, the first electrode 401, and the auxiliary electrode 402 caneach partly serve as external input terminal. An IC chip 420 mountedwith a converter or the like may be provided over the external inputterminals.

As described above, since the lighting device described in thisembodiment includes the light-emitting element described in Embodiment 1or 2 as an EL element, the lighting device can have high luminousefficiency and low power consumption.

Embodiment 4

In this embodiment, a passive matrix light-emitting device manufacturedusing a light-emitting element described in Embodiment 1 or 2 isdescribed with reference to FIGS. 4A and 4B. FIG. 4A is a perspectiveview of the light-emitting device, and FIG. 4B is a cross-sectional viewtaken along the line X-Y in FIG. 4A. In FIGS. 4A and 4B, over asubstrate 951, an EL layer 955 is provided between an electrode 952 andan electrode 956. An end portion of the electrode 952 is covered with aninsulating layer 953. In addition, a partition layer 954 is providedover the insulating layer 953. The sidewalls of the partition layer 954are aslope such that the distance between both sidewalls is graduallynarrowed toward the surface of the substrate. In other words, a crosssection taken along the direction of the short side of the partitionlayer 954 is trapezoidal, and the lower side (a side which is in adirection similar to a plane direction of the insulating layer 953 andis in contact with the insulating layer 953) is shorter than the upperside (a side which is in a direction similar to a plane direction of theinsulating layer 953 and is not in contact with the insulating layer953). By providing the partition layer 954 in this manner, defects ofthe light-emitting element due to static electricity and the like can beprevented. The passive matrix light-emitting device can have highluminous efficiency and low power consumption by including thelight-emitting element in Embodiment 1 or 2.

Embodiment 5

In this embodiment, an active matrix light-emitting device including thelight-emitting element described in Embodiment 1 or 2 is described withreference to FIGS. 5A and 5B.

An example of a light-emitting device in which full color display isachieved with the use of a coloring layer and the like is illustrated inFIGS. 5A and 5B. In FIG. 5A, a substrate 1001, a base insulating film1002, a gate insulating film 1003, gate electrodes 1006, 1007, and 1008,a first interlayer insulating film 1020, a second interlayer insulatingfilm 1021, a peripheral portion 1042, a pixel portion 1040, a drivercircuit portion 1041, first electrodes 1024W, 1024R, 1024C and 1024B oflight-emitting elements, a partition 1025, an EL layer 1028, a secondelectrode 1029 of the light-emitting elements, a sealing substrate 1031,a sealant 1032 a, a sealant 1032 b, and the like are illustrated. Thesealant 1032 b can be mixed with a desiccant. Further, coloring layers(a red coloring layer 1034R, a green coloring layer 1034Q and a bluecoloring layer 1034B) are provided on a transparent base material 1033.Further, a black layer (a black matrix) 1035 may be additionallyprovided. The position of the transparent base material 1033 providedwith the coloring layers and the black layer is aligned and thetransparent base material 1033 is fixed to the substrate 1001. Note thatthe coloring layers and the black layer are covered with an overcoatlayer 1036. In this embodiment, light emitted from part of thelight-emitting layer does not pass through the coloring layers, whilelight emitted from the other part of the light-emitting layer passesthrough the coloring layers. Since light which does not pass through thecoloring layers is white and light which passes through any one of thecoloring layers is red, blue, or green, an image can be displayed usingpixels of the four colors.

The above-described light-emitting device is a light-emitting devicehaving a structure in which light is extracted from the substrate 1001side where the TFTs are formed (a bottom emission structure), but may bea light-emitting device having a structure in which light is extractedfrom the sealing substrate 1031 side (a top emission structure). FIG. 6is a cross-sectional view of a light-emitting device having a topemission structure. In this case, a substrate which does not transmitlight can be used as the substrate 1001. The process up to the step offorming of a connection electrode which connects the TFT and the anodeof the light-emitting element is performed in a manner similar to thatof the light-emitting device having a bottom emission structure. Afterthat, a third interlayer insulating film 1037 is formed to cover anelectrode 1022. This insulating film may have a planarization function.The third interlayer insulating film 1037 can be formed using a materialsimilar to that of the second interlayer insulating film 1021, and canalternatively be formed using any other known material.

The first electrodes 1024W, 1024R, 1024G, and 1024B of thelight-emitting elements each serve as an anode here, but may serve as acathode. Further, in the case of a light-emitting device having a topemission structure as illustrated in FIG. 6, the first electrodes arepreferably reflective electrodes. The EL layer 1028 is formed to have astructure similar to the structure described in Embodiment 1 or 2, withwhich white light emission can be obtained. As the structure with whichwhite light emission can be obtained, in the case where two EL layersare used, a structure with which blue light is obtained from alight-emitting layer in one of the EL layers and orange light isobtained from a light-emitting layer of the other of the EL layers; astructure in which blue light is obtained from a light-emitting layer ofone of the EL layers and red light and green light are obtained from alight-emitting layer of the other of the EL layers; and the like can begiven. Further, in the case where three EL layers are used, red light,green light, and blue light are obtained from respective light-emittinglayers, so that a light-emitting element which emits white light can beobtained. Needless to say, the structure with which white light emissionis obtained is not limited thereto as long as the structure described inEmbodiment 1 or 2 is used.

The coloring layers are each provided in a light path through whichlight from the light-emitting element passes to the outside of thelight-emitting device. In the case of the light-emitting device having abottom emission structure as illustrated in FIG. 5A, the coloring layers1034R, 1034G and 1034B can be provided on the transparent base material1033 and then fixed to the substrate 1001. The coloring layers may beprovided between the gate insulating film 1003 and the first interlayerinsulating film 1020 as illustrated in FIG. 5B. In the case of a topemission structure as illustrated in FIG. 6, sealing can be performedwith the sealing substrate 1031 on which the coloring layers (the redcoloring layer 1034R, the green coloring layer 10344 and the bluecoloring layer 1034B) are provided. The sealing substrate 1031 may beprovided with a black layer (a black matrix) 1035 which is positionedbetween pixels. The coloring layers (the red coloring layer 1034R, thegreen coloring layer 1034Q and the blue coloring layer 1034B) and theblack layer (the black matrix) may be covered with the overcoat layer1036. Note that a light-transmitting substrate is used as the sealingsubstrate 1031.

When voltage is applied between the pair of electrodes of the thusobtained light-emitting element, a white light-emitting region 1044W canbe obtained. In addition, by using the coloring layers, a redlight-emitting region 1044R, a blue light-emitting region 1044B, and agreen light-emitting region 1044G can be obtained. The light-emittingdevice in this embodiment includes the light-emitting element describedin Embodiment 1 or 2; thus, a light-emitting device with low drivingvoltage and low power consumption can be obtained.

Further, although an example in which full color display is performedusing four colors of red, green, blue, and white is shown here, there isno particular limitation and full color display using three colors ofred, green, and blue may be performed.

This embodiment can be freely combined with any of other embodiments.

Embodiment 6

In this embodiment, examples of electronic devices each including thelight-emitting element described in Embodiment 1 or 2 are described. Thelight-emitting element described in Embodiment 1 or 2 has high luminousefficiency and reduced power consumption. As a result, the electronicdevices described in this embodiment can each include a light-emittingportion having reduced power consumption.

Examples of the electronic device to which the above light-emittingelement is applied include television devices (also referred to as TV ortelevision receivers), monitors for computers and the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cell phones or mobile phone devices),portable game machines, portable information terminals, audio playbackdevices, large game machines such as pachinko machines, and the like.Specific examples of these electronic devices are given below.

FIG. 7A illustrates an example of a television device. In the televisiondevice, a display portion 7103 is incorporated in a housing 7101. Inaddition, here, the housing 7101 can be supported on the wall by afixing member 7105. Images can be displayed on the display portion 7103,and in the display portion 7103, the light-emitting elements describedin Embodiment 1 or 2 are arranged in a matrix. The light-emittingelement can have high luminous efficiency. Thus, the television devicehaving the display portion 7103 which is formed using the light-emittingelement can have reduced power consumption.

The television device can be operated with an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

FIG. 7B illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured by using light-emitting elements arrangedin a matrix in the display portion 7203, which are the same as thatdescribed in Embodiment 1 or 2. The computer illustrated in FIG. 7B mayhave a structure illustrated in FIG. 7C. A computer illustrated in FIG.7C is provided with a second display portion 7210 instead of thekeyboard 7204 and the pointing device 7206. The second display portion7210 is a touch screen, and input can be performed by operation ofdisplay for input on the second display portion 7210 with a finger or adedicated pen. The second display portion 7210 can also display imagesother than the display for input. The display portion 7203 may be also atouch screen. Connecting the two screens with a hinge can preventtroubles; for example, the screens can be prevented from being crackedor broken while the computer is being stored or carried. Note that thiscomputer is manufactured using the light-emitting elements each of whichis described in Embodiment 1 or 2 and which are arranged in a matrix inthe display portion 7203. The light-emitting elements can have highluminous efficiency. Therefore, this computer having the display portion7203 which is formed using the light-emitting elements consumes lesspower.

FIG. 7D illustrates a portable game machine, which includes twohousings, a housing 7301 and a housing 7302, which are connected with ajoint portion 7303 so that the portable game machine can be opened orfolded. The housing 7301 incorporates a display portion 7304 includingthe light-emitting elements described in Embodiment 1 or 2 and arrangedin a matrix, and the housing 7302 incorporates a display portion 7305.In addition, the portable game machine illustrated in FIG. 7D includes aspeaker portion 7306, a recording medium insertion portion 7307, an LEDlamp 7308, an input means (an operation key 7309, a connection terminal7310, a sensor 7311 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), or a microphone 7312), and thelike. Needless to say, the structure of the portable game machine is notlimited to the above as long as a display portion including thelight-emitting elements described in Embodiment 1 or 2 which arearranged in a matrix is used for at least either the display portion7304 or the display portion 7305, or both, and the structure can includeother accessories as appropriate. The portable game machine illustratedin FIG. 7D has a function of reading out a program or data stored in astorage medium to display it on the display portion, and a function ofsharing information with another portable game machine by wirelesscommunication. The portable game machine illustrated in FIG. 7D can havea variety of functions without limitation to the above. The portablegame machine having the display portion 7304 can consume less powerbecause the light-emitting elements used in the display portion 7304have high luminous efficiency. Since the light-emitting elements used inthe display portion 7304 has low driving voltage, the portable gamemachine can also be a portable game machine having low driving voltage.Furthermore, since the light-emitting elements used in the displayportion 7304 have a long lifetime, the portable game machine can also bea portable game machine having high reliability.

FIG. 7E illustrates an example of a cellular phone. The cellular phoneis provided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that a mobile phone 7400 hasthe display portion 7402 including the light-emitting elements describedin Embodiment 1 or 2 and arranged in a matrix. The light-emittingelements can have high luminous efficiency. In addition, alight-emitting element driven with a low driving voltage can beprovided. Furthermore, the light-emitting elements can have a longlifetime. Therefore, this mobile phone having the display portion 7402which is formed using the light-emitting elements consumes less power.In addition, a mobile phone driven with a low driving voltage can beprovided. Further, a mobile phone having high reliability can beprovided.

When the display portion 7402 of the mobile phone illustrated in FIG. 7Eis touched with a finger or the like, data can be input into the mobilephone. In this case, operations such as making a call and creating ane-mail can be performed by touching the display portion 7402 with afinger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such ascharacters. The third mode is a display-and-input mode in which twomodes of the display mode and the input mode are combined.

For example, in the case of making a call or creating e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be inputted. In this case,it is preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a detection device including a sensor such as a gyroscope or anacceleration sensor for detecting inclination is provided inside themobile phone, screen display of the display portion 7402 can beautomatically changed by determining the orientation of the mobile phone(whether the mobile phone is placed horizontally or vertically).

The screen modes are switched by touch on the display portion 7402 oroperation with the operation buttons 7403 of the housing 7401. Thescreen modes can be switched depending on the kind of images displayedon the display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed for a certain period while a signal detected by anoptical sensor in the display portion 7402 is detected, the screen modemay be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by thedisplay portion 7402 while in touch with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

Note that the structure described in this embodiment can be combinedwith any of the structures described in Embodiments 1 to 5 asappropriate.

A table lamp 2003 illustrated in FIG. 8A is an example of a lightingdevice including the light-emitting element described in Embodiment 1 or2. The table lamp 2003 includes a housing 2001 and a light source 2002,and the light-emitting element described in Embodiments 1 and 2 is usedfor the light source 2002. FIG. 8B illustrates an example in which thelight-emitting element described in Embodiments 1 and 2 is used for anindoor lighting device 3001 and a television device 3002. The use of thelight-emitting element described in Embodiment 1 or 2 for these lightingdevices can make the lighting devices have reduced power consumption, alarger area, and a reduced thickness.

The light-emitting element described in Embodiment 1 or 2 can also beused for an automobile windshield or an automobile dashboard. FIG. 9Aillustrates one mode in which the light-emitting elements described inEmbodiments 1 and 2 are used for an automobile windshield and anautomobile dashboard.

A display 5000 and a display 5001 are provided in the automobilewindshield in which the light-emitting elements described in Embodiment1 or 2 are incorporated. The light-emitting element described inEmbodiment 1 or 2 can be formed into what is called a see-throughdisplay device, through which the opposite side can be seen, byincluding a first electrode and a second electrode formed of electrodeshaving light-transmitting properties. Such see-through display devicescan be provided even in the automobile windshield, without hindering thevision. Note that in the case where a transistor for driving or the likeis provided, a transistor having a light-transmitting property, such asan organic transistor using an organic semiconductor material or atransistor using an oxide semiconductor, is preferably used.

A display 5002 is provided in a pillar portion in which thelight-emitting elements described in Embodiment 1 or 2 are incorporated.The display 5002 can compensate for the view hindered by the pillarportion by showing an image taken by an imaging unit provided in the carbody. Similarly, a display 5003 provided in the dashboard can compensatefor the view hindered by the car body by showing an image taken by animaging unit provided in the outside of the car body, which leads toelimination of blind areas and enhancement of safety. Showing an imageso as to compensate for the area which a driver cannot see makes itpossible for the driver to confirm safety easily and comfortably.

A display 5004 and a display 5005 can provide a variety of kinds ofinformation such as navigation data, a speedometer, a tachometer, amileage, a fuel meter, a gearshift indicator, and air-condition setting.The content or layout of the display can be changed freely by a user asappropriate. Note that such information can also be shown by thedisplays 5000 to 5003. The displays 5000 to 5005 can also be used aslighting devices.

Further, as illustrated in FIG. 9B, the light-emitting element describedin Embodiment 1 or 2 may be used for a display portion of the licenseplate 5011. Accordingly, the visibility of the license plate 5011 can beimproved.

As illustrated in FIG. 9C, the light-emitting element described inEmbodiment 1 or 2 may be used for hands 5021 or a display portion 5022of a watch. Accordingly, without a radioactive substance such astritium, which is used in a conventional light-emitting watch, thevisibility of the watch in a dark place can be improved.

As described above, the application range of the light-emitting devicehaving the light-emitting element described in Embodiment 1 or 2 is wideso that this light-emitting device can be applied to electronic devicesin a variety of fields. By using the light-emitting element described inEmbodiment 1 or 2, an electronic device having low power consumption canbe obtained.

Example 1

In this example, a light-emitting element in which a mixture of athermally activated delayed fluorescent substance and a fluorescentmaterial is used for a light-emitting layer and a comparisonlight-emitting element in which a mixture of a material which does notemit thermally activated delayed fluorescence and a fluorescent materialis used for a light-emitting layer were actually formed to be comparedwith each other. The comparison results are described with reference toFIG. 11 to FIG. 15.

Hereinafter, the light-emitting element 1 is a light-emitting element inwhich the thermally activated delayed fluorescent substance and thefluorescent material are mixed to be used for a light-emitting layer.The comparison light-emitting element 1 is a light-emitting element inwhich the material which does not emit thermally activated delayedfluorescence and the fluorescent material are mixed to be used for alight-emitting layer.

The fluorescent material which is used for the light-emitting element 1and the comparison light-emitting element 1 is 5,6,11,12-tetraphenylnaphthacene (trivial name: rubrene).

As the thermally activated delayed fluorescent substance in thelight-emitting element 1, two kinds of organic compounds which form anexciplex were used. Specifically,2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II) was used as the first organic compound, and2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF) was used as the second organic compound.

As the material which does not emit thermally activated delayedfluorescence in the comparison light-emitting element 1,2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II) was used. That is, as the material which does not emitthermally activated delayed fluorescence, the first organic compound inthe light-emitting element 1 was only used.

Chemical formulae of materials used in this example are shown below.

Methods for manufacturing the light-emitting element 1 and thecomparison light-emitting element 1 are described below.

(Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate 1100 by a sputtering method, so that afirst electrode 1101 functioning as an anode was formed. The thicknessthereof was 110 nm and the electrode area was 2 mm×2 mm (see FIG. 11).

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Then, the substrate 1100 over which the first electrode 1101 was formedwas fixed to a substrate holder provided in the vacuum evaporationapparatus so that the surface on which the first electrode 1101 wasformed faced downward. The pressure in the vacuum evaporation apparatuswas reduced to about 10⁻⁴ Pa. After that, over the first electrode 1101,1,3,5-tri(dibenzothiophen-4-yl)-benzene (abbreviated as DBT3P-II) andmolybdenum oxide were deposited by co-evaporation, so that ahole-injection layer 1111 was formed. The thickness of thehole-injection layer 1111 was set to 40 nm, and the weight ratio ofDBT3P-II to molybdenum oxide was adjusted to 1:0.5.

Next, a film of BPAFLP (abbreviation) was formed to a thickness of 20 nmover the hole-injection layer 1111 to form a hole-transport layer 1112.

2mDBTPDBq-II (abbreviation), PCASF (abbreviation), and rubrene weredeposited by co-evaporation so that a light-emitting layer 1113 isformed over the hole-transport layer 1112. The weight ratio of2mDBTPDBq-II to PCASF and rubrene was adjusted to be 0.8:0.2:0.01(=2mDBTPDBq-II:PCASF:rubrene). The thickness of the light-emitting layer1113 was set to 30 nm.

Next, over the light-emitting layer 1113, a film of 2mDBTPDBq-II(abbreviation) was formed to a thickness of 20 nm to form a firstelectron-transport layer 1114 a.

Next, a film of bathophenanthroline (abbreviation: BPhen) was formed toa thickness of 20 nm over the first electron-transport layer 1114 a toform a second electron-transport layer 1114 b.

Lithium fluoride (LiF) was deposited over the second electron-transportlayer 1114 b by evaporation to a thickness of 1 nm, so that anelectron-injection layer 1115 was formed.

Lastly, a 200 nm thick film of aluminum was deposited by evaporation toform a second electrode 1103 functioning as a cathode. Thus,Light-emitting Element 1 of this example was fabricated.

(Comparison Light-Emitting Element 1)

The light-emitting layer 1113 of the comparison light-emitting element 1was deposited by co-evaporation of 2mDBTPDBq-II (abbreviation) andrubrene. The weight ratio of 2mDBTPDBq-II (abbreviation) to rubrene wasadjusted to be 1:0.01 (=2mDBTPDBq-II:rubrene). The thickness of thelight-emitting layer 1113 was set to 30 run. Components other than thelight-emitting layer 1113 were manufactured in a manner similar to thatof the light-emitting element 1.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Element structures of the light-emitting element 1 and the comparisonlight-emitting element 1 obtained as described above are shown in Table1.

TABLE 1 Light-emitting Element 1 Comparison Light- emitting Element 1Electron-injection Layer LiF 1 nm LiF 1 nm Electron-transport LayerBPhen 20 nm BPhen 20 nm 2mDBTPDBq-II 20 nm 2mDBTPDBq-II 20 muLight-emitting Layer 2mDBTPDBq-II :PCASF :Rubrene (= 0.8:0.2:0.01) 30 nm2mDBTPDBq-II :Rubrene (= 0.8:0.01) 30 nm Hole-transport Layer BPAFLP 20nm BPAFLP 20 nm Hole-injection Layer DBT3P-II:MoOx (= 1:0.5) 20 nmDBT3P-II:MoOx (= 1:0.5) 20 nm *First Electrode: Indium Tin OxideContaining Silicon Oxide 110 nm Second Electrode: Al 200 am

These light-emitting elements were each sealed in a glove box containinga nitrogen atmosphere so as not to be exposed to the air. Then,operation characteristics of these light-emitting elements weremeasured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 12 shows voltage-luminance characteristics of the light-emittingelement 1 and the comparison light-emitting element 1. In FIG. 12, thehorizontal axis represents voltage (V) and the vertical axis representsluminance (cd/m²). FIG. 13 shows luminance-current efficiencycharacteristics. In FIG. 13, the horizontal axis indicates luminance(cd/m²) and the vertical axis indicates current efficiency (cd/A). FIG.14 shows luminance-power efficiency characteristics thereof. In FIG. 14,the horizontal axis represents luminance (cd/m²), and the vertical axisrepresents power efficiency (lm/W). In addition, FIG. 15 showsluminance-external quantum efficiency characteristics thereof. In FIG.15, the horizontal axis represents luminance (cd/m²) and the verticalaxis represents external quantum efficiency (%).

Further, Table 2 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 1 and the comparison light-emitting element 1 ata luminance of around 1000 cd/m².

TABLE 2 Light-emitting Element 1 Comparison Light- emitting Element 1Voltage (V) 3.6 4.2 Current Density (mA/cm²) 6.5 10.5 Chromaticity (x,y) (0.47, 0.52) (0.46, 0.50) Luminance (cd/m²) 950 1130 CurrentEfficiency (cd/A) 15 11 Power Efficiency (lm/W) 13 8 External Quantum4.3 3.2 Efficiency (%)

As shown in Table 2, CIE chromaticity coordinates of the light-emittingelement 1 at luminance of around 1000 cd/m² were (x,y)=(0.47, 0.52), andCIE chromaticity coordinates of the comparison light-emitting element 1at luminance of around 1000 cd/m² were (x,y)=(0.46, 0.50). These resultsshow that the light-emitting element 1 and the comparison light-emittingelement 1 emit yellow light derived from rubrene.

As apparent from Table 2, FIG. 12, FIG. 13, FIG. 14, and FIG. 15, thelight-emitting element 1 has a low threshold voltage at which thefluorescent material starts to emit light (light emission startvoltage), high current efficiency, high power efficiency, and highexternal quantum efficiency as compared to the comparison light-emittingelement 1. Since 2mDBTPDBq-II and PCASF which are used for thelight-emitting layer 1113 form an exciplex, a singlet excited state isformed from part of a triplet excited state of the exciplex in thelight-emitting layer 1113. The reason why the luminous efficiency wasimproved is considered to be because of the energy transfer of thissinglet excited state of the exciplex to the singlet excited state ofthe fluorescent material. The reason why the light emission startvoltage was lowered is considered to be because of the formation of thisexciplex.

Example 2

In this example, as in Example 1, a light-emitting element in which amixture of a thermally activated delayed fluorescent substance and afluorescent material is used for a light-emitting layer and a comparisonlight-emitting element in which a mixture of a material which does notemit thermally activated delayed fluorescence and a fluorescent materialis used for a light-emitting layer are manufactured to be compared witheach other. The comparison results are described with reference to FIG.16 to FIG. 22.

Hereinafter, the light-emitting element 2 is a light-emitting element inwhich the thermally activated delayed fluorescent substance and thefluorescent material are mixed to be used for a light-emitting layer.The comparison light-emitting element 2 is a light-emitting element inwhich the material which does not emit thermally activated delayedfluorescence and the fluorescent material are mixed to be used for alight-emitting layer.

The fluorescent material which is used for the light-emitting element 2and the comparison light-emitting element 2 is 5,6,11,12-tetraphenylnaphthacene (trivial name: rubrene).

As the thermally activated delayed fluorescent substance in thelight-emitting element 2, two kinds of organic compounds which form anexciplex were used. Specifically,4,6-bis[3-(9H-carbazol-9-yl)-phenyl]pyrimidine (abbreviation:4,6mCzP2Pm) was used as the first organic compound, andN-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF) was used as the second organic compound.

As the material which does not emit thermally activated delayedfluorescence in the comparison light-emitting element 2,4,6-bis[3-(9H-carbazol-9-yl)-phenyl]pyrimidine (abbreviation:4,6mCzP2Pm) was used. That is, as the material which does not emitthermally activated delayed fluorescence, the first organic compound inthe light-emitting element 2 was only used.

Chemical formulae of materials used in this example are shown below.

Methods for manufacturing the light-emitting element 2 and thecomparison light-emitting element 2 are described below.

(Light-Emitting Element 2)

First, the first electrode 1101, the hole-injection layer 1111, and thehole-transport layer 1112 were formed over the glass substrate 1100using a material and a method similar to those of the light-emittingelement 1.

Next, 4,6mCzP2Pm (abbreviation), PCBBiF (abbreviation), and rubrene weredeposited by co-evaporation, so that the light-emitting layer 1113 wasformed over the hole-transport layer 1112. The weight ratio of4,6mCzP2Pm to PCBBiF and rubrene was adjusted to be 0.8:0.2:0.0075(=4,6mCzP2Pm:PCBBiF:rubrene). The thickness of the light-emitting layer1113 was set to 40 nm.

Further, over the light-emitting layer 1113, a film of 4,6mCzP2Pm(abbreviation) was formed to a thickness of 10 nm to form the firstelectron-transport layer 1114 a.

Next, a film of bathophenanthroline (abbreviation: BPhen) was formed toa thickness of 15 nm over the first electron-transport layer 1114 a toform the second electron-transport layer 1114 b.

Further, the electron-injection layer 1115 and a second electrode wereformed using a material and a condition similar to the material and thecondition for the electron-injection layer 1115 and the second electrodeof the light-emitting element 1, so that the light-emitting element 2 inthis example was formed.

(Comparison Light-Emitting Element 2)

The light-emitting layer 1113 of the comparison light-emitting element 2was deposited by co-evaporation of 4,6mCzP2Pm (abbreviation) andrubrene. The weight ratio of 4,6mCzP2Pm and rubrene was adjusted to be1:0.005 (=4,6mCzP2Pm: rubrene). The thickness of the light-emittinglayer 1113 was set to 40 nm. Components other than the light-emittinglayer 1113 were manufactured in a manner similar to that of thelight-emitting element 2.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Element structures of the light-emitting element 2 and the comparisonlight-emitting element 2 obtained as described above are shown in Table3.

TABLE 3 Light-emitting Element 2 Comparison Light- emitting Element 2Electron-injection Layer LiF 1 nm LiF 1 nm Electron-transport LayerBPhen 15 nm BPhen 15 nm 4, 6mCzP2Pm 10 nm 4, 6mCzPZPm 10 nmLight-emitting Layer 4, 6mCzP2Pm :PCBBIF :Rubrene (= 0:8:0.2:0.0075) 40nm 4, 6mCzP2Pm Rubrene (= 1:0.005) 40 nm Hole-transport Layer BPAFLP 20nm BPAFLP 20 nm Hole-injection Layer DBT3P-II:MoOx (= 1:0.5) 20 nmDBT3P-II:MoOx (= 1:0.5) 20 nm *First Electrode: Indium Tin OxideContaining Silicon Oxide 110 nm Second Electrode: Al 200 nm

These light-emitting elements were each sealed in a glove box containinga nitrogen atmosphere so as not to be exposed to the air. Then,operation characteristics of these light-emitting elements weremeasured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 16 shows voltage-luminance characteristics of the light-emittingelement 2 and the comparison light-emitting element 2. In FIG. 16, thehorizontal axis represents voltage (V) and the vertical axis representsluminance (cd/m²). FIG. 17 shows luminance-current efficiencycharacteristics. In FIG. 17, the horizontal axis indicates luminance(cd/m²) and the vertical axis indicates current efficiency (cd/A). FIG.18 shows voltage-current characteristics. In FIG. 18, the horizontalaxis indicates voltage (V) and the vertical axis indicates current (mA).FIG. 19 shows luminance-power efficiency characteristics thereof. InFIG. 19, the horizontal axis represents luminance (cd/m²), and thevertical axis represents power efficiency (lm/W). In addition, FIG. 20shows luminance-external quantum efficiency characteristics thereof. InFIG. 20, the horizontal axis represents luminance (cd/m²) and thevertical axis represents external quantum efficiency (%).

Further, Table 4 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 2 and the comparison light-emitting element 2 ata luminance of around 1000 cd/m².

TABLE 4 Light-emitting element 2 Comparison Light- emitting Element 2Voltage (V) 3.5 4.2 Current Density (mA/cm²) 4.4 9.1 Chromaticity (x, y)(0.47, 0.52) (0.47. 0.50) Luminance (cd/m²) 972 1076 Current Efficiency(cd/A) 22 12 Power Efficiency (lm/W) 20 9 External Quantum Efficiency(%) 6.5 3.6

As shown in Table 4, CIE chromaticity coordinates of the light-emittingelement 2 at luminance of around 1000 cd/m² were (x,y)=(0.47, 0.52), andCIE chromaticity coordinates of the comparison light-emitting element 2at luminance of around 1000 cd/m² were (x,y)=(0.47, 0.50).

FIG. 21 shows emission spectra of the light-emitting element 2 and thecomparison light-emitting element 2 which were obtained by applying acurrent of 0.1 mA. In FIG. 21, the vertical axis represents emissionintensity (arbitrary unit) and the horizontal axis represents wavelength(nm). The emission intensity is shown as a value relative to the maximumemission intensity assumed to be 1. As shown in FIG. 21, thelight-emitting element 2 and the comparison light-emitting element 2each show a spectrum having a maximum emission wavelength at around 558nm, which is derived from rubrene. This and the results of Table 4 showthat the light-emitting element 2 and the comparison light-emittingelement 2 emit yellow light.

The reliability tests were carried out, and the results thereof areshown in FIG. 22. In the reliability tests, the light-emitting element 2and the comparison light-emitting element 2 were driven under theconditions where the initial luminance was set to 5000 cd/m² and thecurrent density was constant. FIG. 22 shows a change in normalizedluminance where the initial luminance is 100%.

As apparent from Table 4, FIG. 16 to FIG. 22, the light-emitting element2 has a low threshold voltage at which the fluorescent material startsto emit light (light emission start voltage), high current efficiency,high power efficiency, and high external quantum efficiency as comparedto the comparison light-emitting element 2. The light-emitting element 2is a highly-reliable light-emitting element which shows a smallluminance decrease relative to driving time.

Since 4,6mCzP2Pm and PCBBiF which are used for the light-emitting layer1113 form an exciplex, a singlet excited state is formed from part of atriplet excited state of the exciplex in the light-emitting layer 1113.The reason why the luminous efficiency was improved is considered to bebecause of the energy transfer of this singlet excited state of theexciplex to the singlet excited state of the fluorescent material. Thereason why the light emission start voltage was lowered is considered tobe because of the formation of this exciplex.

Example 3

In this example, as in Example 1, a light-emitting element in which amixture of a thermally activated delayed fluorescent substance and afluorescent material is used for a light-emitting layer and a comparisonlight-emitting element in which a mixture of a material which does notemit thermally activated delayed fluorescence and a fluorescent materialis used for a light-emitting layer are manufactured to be compared witheach other. The comparison results are described with reference to FIG.23 to FIG. 29.

Hereinafter, the light-emitting element 3 is a light-emitting element inwhich the thermally activated delayed fluorescent substance and thefluorescent material are mixed to be used for a light-emitting layer.The comparison light-emitting element 3 is a light-emitting element inwhich the material which does not emit thermally activated delayedfluorescence and the fluorescent material are mixed to be used for alight-emitting layer.

The fluorescent material which is used for the light-emitting element 3and the comparison light-emitting element 3 is coumarin 6 (trivialname).

As the thermally activated delayed fluorescent substance in thelight-emitting element 3, the two kinds of organic compounds which forman exciplex and which are the same as the organic compounds in Example 2were used. Specifically, 4,6-bis[3-(9H-carbazol-9-yl)-phenyl]pyrimidine(abbreviation: 4,6mCzP2Pm) was used as the first organic compound, andN-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluorene-2-amine (abbreviation: PCBBiF) was used as the second organiccompound.

As the material which does not emit thermally activated delayedfluorescence in the comparison light-emitting element 3,4,6-bis[3-(9H-carbazol-9-yl)-phenyl]pyrimidine (abbreviation:4,6mCzP2Pm), which is the same as the one in Example 2, was used. Thatis, as the material which does not emit thermally activated delayedfluorescence, the first organic compound in the light-emitting element 3was only used.

For the chemical formulae of the materials used in this example, thechemical formulae in Example 2 can be referred to.

Methods for manufacturing the light-emitting element 3 and thecomparison light-emitting element 3 are described below.

(Light-Emitting Element 3)

First, the first electrode 1101, the hole-injection layer 1111, and thehole-transport layer 1112 were formed over the glass substrate 1100using a material and a method similar to those of the light-emittingelement 1.

Next, 4,6mCzP2Pm (abbreviation), PCBBiF (abbreviation), and coumarin 6were deposited by co-evaporation, so that the light-emitting layer 1113was formed over the hole-transport layer 1112. The weight ratio of4,6mCzP2Pm to PCBBiF and coumarin 6 was adjusted to be 0.8:0.2:0.005(=4,6mCzP2Pm:PCBBiF:coumarin 6). The thickness of the light-emittinglayer 1113 was set to 40 nm.

Further, over the light-emitting layer 1113, a film of 4,6mCzP2Pm(abbreviation) was formed to a thickness of 10 nm to form the firstelectron-transport layer 1114 a.

Next, a film of bathophenanthroline (abbreviation: BPhen) was formed toa thickness of 15 nm over the first electron-transport layer 1114 a toform the second electron-transport layer 1114 b.

Further, the electron-injection layer 1115 and a second electrode wereformed using a material and a condition similar to the material and thecondition for the electron-injection layer 1115 and the second electrodeof the light-emitting element 1, so that the light-emitting element 3 inthis example was formed.

(Comparison Light-Emitting Element 3)

The light-emitting layer 1113 of the comparison light-emitting element 3was deposited by co-evaporation of 4,6mCzP2Pm (abbreviation) andcoumarin 6. The weight ratio of 4,6mCzP2Pm and coumarin 6 was adjustedto be 1:0.005 (=4,6mCzP2Pm:coumarin 6). The thickness of thelight-emitting layer 1113 was set to 40 nm. Components other than thelight-emitting layer 1113 were manufactured in a manner similar to thatof the light-emitting element 3.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Element structures of the light-emitting element 3 and the comparisonlight-emitting element 3 obtained as described above are shown in Table5.

TABLE 5 Light-emitting Element 3 Comparison Light- emitting Element 3Electron-injection Layer LiF 1 nm LiF 1 nm Electron-transport LayerBPhen BPhen 15 nm 15 nm 4, 6mCzP2Pm 10 nm 4, 6mCzP2Pm 10 nmLight-emitting Layer 4, 6mCzP2Pm :PCBBIF :Coumarin6 (= 0.8:0.2:0.005) 40nm 4, 6mCzP2Pm :Coumarin6 (= 1:0.005) 40 nm Hole-transport Layer BPAFLP20 nm BPAFLP 20 nm Hole-injection Layer DBT3P-II:MoOx (= 1:0.5) 20 nmDBT3P-II:MoOx (= 1:0.5) 20 nm *First Electrode: Indium Tin OxideContaining Silicon Oxide 110 nm Second Electrode: Al 200 nm

These light-emitting elements were each sealed in a glove box containinga nitrogen atmosphere so as not to be exposed to the air. Then,operation characteristics of these light-emitting elements weremeasured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 23 shows voltage-luminance characteristics of the light-emittingelement 3 and the comparison light-emitting element 3. In FIG. 23, thehorizontal axis represents voltage (V) and the vertical axis representsluminance (cd/m²). FIG. 24 shows luminance-current efficiencycharacteristics. In FIG. 24, the horizontal axis indicates luminance(cd/m²) and the vertical axis indicates current efficiency (cd/A). FIG.25 shows voltage-current characteristics. In FIG. 25, the horizontalaxis indicates voltage (V) and the vertical axis indicates current (mA).FIG. 26 shows luminance-power efficiency characteristics thereof. InFIG. 26, the horizontal axis represents luminance (cd/m²), and thevertical axis represents power efficiency (lm/W). In addition, FIG. 27shows luminance-external quantum efficiency characteristics thereof. InFIG. 27, the horizontal axis represents luminance (cd/m²) and thevertical axis represents external quantum efficiency (%).

Further, Table 6 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 3 and the comparison light-emitting element 3 ata luminance of around 1000 cd/m².

TABLE 6 Light-emitting Element 3 Comparison Light- emitting Element 3Voltage (V) 3.5 3.9 Current Density (mA/cm²) 7.6 10.8 Chromaticity (x,y) (0.28, 0.60) (0.26, 0.58) Luminance (cd/m²) 1087 865 CurrentEfficiency (cd/A) 14 8 Power Efficiency (lm/W) 13 6 External QuantumEfficiency (%) 4.5 2.6

As shown in Table 6, CIE chromaticity coordinates of the light-emittingelement 3 at luminance of around 1000 cd/m² were (x,y)=(0.28, 0.60), andCIE chromaticity coordinates of the comparison light-emitting element 3at luminance of around 1000 cd/m² were (x,y)=(0.26, 0.58).

FIG. 28 shows emission spectra of the light-emitting element 3 and thecomparison light-emitting element 3 which were obtained by applying acurrent of 0.1 mA. In FIG. 28, the vertical axis represents emissionintensity (arbitrary unit) and the horizontal axis represents wavelength(nm). The emission intensity is shown as a value relative to the maximumemission intensity assumed to be 1. As shown in FIG. 28, thelight-emitting element 3 and the comparison light-emitting element 3each show a spectrum having a maximum emission wavelength at around 500nm, which is derived from coumarin 6. This and the results of Table 6show that the light-emitting element 3 and the comparison light-emittingelement 3 emit green light.

The reliability tests were carried out, and the results thereof is shownin FIG. 29. In the reliability tests, the light-emitting element 3 andthe comparison light-emitting element 3 were driven under the conditionswhere the initial luminance was set to 5000 cd/m² and the currentdensity was constant. FIG. 29 shows a change in normalized luminancewhere the initial luminance is 100%.

As apparent from Table 6, FIG. 23 to FIG. 29, the light-emitting element3 has a low threshold voltage at which the fluorescent material startsto emit light (light emission start voltage), high current efficiency,high power efficiency, and high external quantum efficiency as comparedto the comparison light-emitting element 3. The light-emitting element 3is a highly-reliable light-emitting element which shows a smallluminance decrease relative to driving time.

Since 4,6mCzP2Pm and PCBBiF which are used for the light-emitting layer1113 form an exciplex, a singlet excited state is formed from part of atriplet excited state of the exciplex in the light-emitting layer 1113.The reason why the luminous efficiency was improved is considered to bebecause of the energy transfer of this singlet excited state of theexciplex to the singlet excited state of the fluorescent material. Thereason why the light emission start voltage was lowered is considered tobe because of the formation of this exciplex.

Example 4

In this example, as in Example 1, a light-emitting element in which amixture of a thermally activated delayed fluorescent substance and afluorescent material is used for a light-emitting layer and a comparisonlight-emitting element in which a mixture of a material which does notemit thermally activated delayed fluorescence and a fluorescent materialis used for a light-emitting layer are manufactured to be compared witheach other. The comparison results are described with reference to FIG.30 to FIG. 35.

Hereinafter, the light-emitting element 4 is a light-emitting element inwhich the thermally activated delayed fluorescent substance and thefluorescent material are mixed to be used for a light-emitting layer.The comparison light-emitting element 4 is a light-emitting element inwhich the material which does not emit thermally activated delayedfluorescence and the fluorescent material are mixed to be used for alight-emitting layer.

The fluorescent material which is used for the light-emitting element 3and the comparison light-emitting element 3 is{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB).

As the thermally activated delayed fluorescent substance in thelight-emitting element 4, two kinds of organic compounds which form anexciplex and are the same as the organic compounds in Example 2 wereused. Specifically, 4,6-bis[3-(9H-carbazol-9-yl)-phenyl]pyrimidine(abbreviation: 4,6mCzP2Pm) was used as the first organic compound, andN-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluorene-2-amine (abbreviation: PCBBiF) was used as the second organiccompound.

As the material which does not emit thermally activated delayedfluorescence in the comparison-light-emitting element 4,6-bis[3-(9H-carbazol-9-yl)-phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm),which is the same as the one in Example 2, was used. That is, as thematerial which does not emit thermally activated delayed fluorescence,the first organic compound in the light-emitting element 4 was onlyused.

For the chemical formulae of the materials used in this example, thechemical formulae in Example 2 can be referred to.

Methods for manufacturing the light-emitting-element 4 and thecomparison light-emitting element 4 are described below.

(Light-Emitting Element 4)

First, the first electrode 1101, the hole-injection layer 1111, and thehole-transport layer 1112 were formed over the glass substrate 1100using a material and a method similar to those of the light-emittingelement 1.

Next, 4,6mCzP2Pm (abbreviation), PCBBiF (abbreviation), and DCJTB(abbreviation) were deposited by co-evaporation, so that thelight-emitting layer 1113 was formed over the hole-transport layer 1112.The weight ratio of 4,6mCzP2Pm to PCBBiF and DCJTB was adjusted to be0.8:0.2:0.005 (=4,6mCzP2Pm:PCBBiF:DCJTB). The thickness of thelight-emitting layer 1113 was set to 40 nm.

Further, over the light-emitting layer 1113, a film of 4,6mCzP2Pm(abbreviation) was formed to a thickness of 10 nm to form the firstelectron-transport layer 1114 a.

Next, a film of bathophenanthroline (abbreviation: BPhen) was formed toa thickness of 15 nm over the first electron-transport layer 1114 a toform the second electron-transport layer 1114 b.

Further, the electron-injection layer 1115 and a second electrode wereformed using a material and a condition similar to the material and thecondition for the electron-injection layer 1115 and the second electrodeof the light-emitting element 1, so that the light-emitting element 4 inthis example was formed.

(Comparison Light-Emitting Element 4)

The light-emitting layer 1113 of the comparison light-emitting element 3was deposited by co-evaporation of 4,6mCzP2Pm (abbreviation) and DCJTB(abbreviation). The weight ratio of 4,6mCzP2Pm and DCJTB was adjusted tobe 1:0.005 (=4,6mCzP2Pm:DCJTB). The thickness of the light-emittinglayer 1113 was set to 40 nm. Components other than the light-emittinglayer 1113 were manufactured in a manner similar to that of thelight-emitting element 4.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Element structures of the light-emitting element 4 and the comparisonlight-emitting element 4 obtained as described above are shown in Table7.

TABLE 7 Light-emitting Element 4 Comparison Light- emitting Element 4Electron-injection Layer LiF 1 nm LiF 1 nm Electron-transport LayerBPhen BPhen 15 nm 15 nm 4, 6mCzP2Pm 10 nm 4, 6mCzP2Pm 10 nmLight-emitting Layer 4, 6mCzP2Pm :PCBBiF :DCJTB (= 0.8:0.2:0.005) 40 nm4, 6mCzP2Pm :DCJTB (= 1:0.005) 40 nm Hole-transport Layer BPAFLP 20 nmBPAFLP 20 nm Hole-injection Layer DBT3P-II:MoOx (= 1:0.5) 20 nmDBT3P-II:MoOx (= 1:0.5) 20 nm *First Electrode: Indium Tin OxideContaining Silicon Oxide 110 nm Second Electrode: Al 200 nm

These light-emitting elements were each sealed in a glove box containinga nitrogen atmosphere so as not to be exposed to the air. Then,operation characteristics of these light-emitting elements weremeasured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 30 shows voltage-luminance characteristics of the light-emittingelement 3 and the comparison light-emitting element 3. In FIG. 30, thehorizontal axis represents voltage (V) and the vertical axis representsluminance (cd/m²). FIG. 31 shows luminance-current efficiencycharacteristics. In FIG. 31, the horizontal axis indicates luminance(cd/m²) and the vertical axis indicates current efficiency (cd/A). FIG.32 shows voltage-current characteristics. In FIG. 32, the horizontalaxis indicates voltage (V) and the vertical axis indicates current (mA).FIG. 33 shows luminance-power efficiency characteristics thereof. InFIG. 33, the horizontal axis represents luminance (cd/m²), and thevertical axis represents power efficiency (lm/W). In addition, FIG. 34shows luminance-external quantum efficiency characteristics thereof. InFIG. 34, the horizontal axis represents luminance (cd/m²) and thevertical axis represents external quantum efficiency (%).

Further, Table 8 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), powerefficiency (lm/W), and external quantum efficiency (%) of each of thelight-emitting element 4 and the comparison light-emitting element 4 ata luminance of around 1000 cd/m².

TABLE 8 Light-emitting Element 4 Comparison Light- emitting Element 4Voltage (V) 4.4 6.2 Current Density (mA/cm²) 8.2 16.8 Chromaticity (x,y) (0.57, 0.43) (0.56, 0.41) Luminance (cd/m²) 1075 976 CurrentEfficiency (cd/A) 13 6 Power Efficiency (lm/W) 9 3 External QuantumEfficiency (%) 5.4 2.5

As shown in Table 8, CIE chromaticity coordinates of the light-emittingelement 4 at luminance of around 1000 cd/m² were (x,y)=(0.57, 0.43), andCIE chromaticity coordinates of the comparison light-emitting element 4at luminance of around 1000 cd/m² were (x,y)=(0.56, 0.41).

FIG. 35 shows emission spectra of the light-emitting element 4 and thecomparison light-emitting element 4 which were obtained by applying acurrent of 0.1 mA. In FIG. 35, the vertical axis represents emissionintensity (arbitrary unit) and the horizontal axis represents wavelength(nm). The emission intensity is shown as a value relative to the maximumemission intensity assumed to be 1. As shown in FIG. 35, thelight-emitting element 4 and the comparison light-emitting element 4each show a spectrum having a maximum emission wavelength at around 595nm, which is derived from DCJTB. This and the results of Table 8 showthat the light-emitting element 4 and the comparison light-emittingelement 4 emit yellow light.

The reliability tests were carried out, and the results thereof is shownin FIG. 36. In the reliability tests, the light-emitting element 4 andthe comparison light-emitting element 4 were driven under the conditionswhere the initial luminance was set to 5000 cd/m² and the currentdensity was constant. FIG. 36 shows a change in normalized luminancewhere the initial luminance is 100%.

As apparent from Table 8, FIG. 30 to FIG. 36, the light-emitting element4 has a low threshold voltage at which the fluorescent material startsto emit light (light emission start voltage), high current efficiency,high power efficiency, and high external quantum efficiency as comparedto the comparison light-emitting element 4. The light-emitting element 4is a highly-reliable light-emitting element which shows a smallluminance decrease relative to driving time.

Since 4,6mCzP2Pm and PCBBiF which are used for the light-emitting layer1113 form an exciplex, a singlet excited state is formed from part of atriplet excited state of the exciplex in the light-emitting layer 1113.The reason why the luminous efficiency was improved is considered to bebecause of the energy transfer of this singlet excited state of theexciplex to the singlet excited state of the fluorescent material. Thereason why the light emission start voltage was lowered is considered tobe because of the formation of this exciplex.

This application is based on Japanese Patent Application serial No.2012-172830 filed with Japan Patent Office on Aug. 3, 2012, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A light-emitting device comprising: a first compoundand a second compound in a light-emitting layer, wherein in the firstcompound, a difference between levels of a singlet excited state and atriplet excited state is 0.2 eV or less, wherein the second compoundcomprises a fluorescent material, wherein the first compound exhibitsthermally activated delayed fluorescence, wherein a peak wavelength ofan emission spectrum of the first compound is longer than or equal to apeak wavelength of an absorption band on the lowest energy side in anabsorption spectrum of the fluorescent material, wherein an energy istransferred to from a level of the singlet excited state of the firstcompound to a level of a single exited state of the fluorescentmaterial, and wherein the fluorescent material emits fluorescence.
 3. Alight-emitting device comprising: a first compound and a second compoundin a light-emitting layer, wherein in the first compound, a differencebetween levels of a singlet excited state and a triplet excited state is0.2 eV or less, wherein the second compound comprises a fluorescentmaterial, wherein the first compound exhibits thermally activateddelayed fluorescence, wherein an emission spectrum of the first compoundoverlaps with an absorption band on the lowest energy side in anabsorption spectrum of the fluorescent material, wherein a peakwavelength of the emission spectrum of the first compound is longer thanor equal to a peak wavelength of the absorption band on the lowestenergy side in the absorption spectrum of the fluorescent material, andwherein the fluorescent material emits fluorescence.
 4. A light-emittingdevice comprising: a thermally activated delayed fluorescent substanceand a fluorescent material in a light-emitting layer, wherein thethermally activated delayed fluorescent substance comprises a firstorganic compound and a second organic compound, wherein the firstorganic compound and the second organic compound form an exciplex,wherein a peak wavelength of an emission spectrum of the thermallyactivated delayed fluorescent substance is longer than or equal to apeak wavelength of an absorption band on the lowest energy side in anabsorption spectrum of the fluorescent material, and wherein thefluorescent material emits fluorescence.
 5. The light-emitting deviceaccording to claim 2, wherein the first compound is configured toexhibit reverse intersystem crossing.
 6. The light-emitting deviceaccording to claim 2, wherein a difference between the peak wavelengthof the emission spectrum of the first compound and a peak wavelength ofan emission spectrum of the fluorescent material is 30 nm or less. 7.The light-emitting device according to claim 2, further comprising: apair of electrodes; and an EL layer between the pair of electrodes,wherein the EL layer comprises the light-emitting layer.
 8. Thelight-emitting device according to claim 3, wherein an energy istransferred from the level of the singlet excited state of the firstcompound to a level of a singlet excited state of the fluorescentmaterial.
 9. The light-emitting device according to claim 3, wherein adifference between the peak wavelength of the emission spectrum of thefirst compound and a peak wavelength of an emission spectrum of thefluorescent material is 30 nm or less.
 10. The light-emitting deviceaccording to claim 3, further comprising: a pair of electrodes; and anEL layer between the pair of electrodes, wherein the EL layer comprisesthe light-emitting layer.
 11. The light-emitting device according toclaim 4, wherein the emission spectrum of the thermally activateddelayed fluorescent substance overlaps with the absorption band on thelowest energy side in the absorption spectrum of the fluorescentmaterial.
 12. The light-emitting device according to claim 4, wherein adifference in energy between the peak wavelength of the absorption bandon the lowest energy side of the fluorescent material and a peakwavelength of the exciplex is 0.2 eV or less.
 13. The light-emittingdevice according to claim 4, wherein one of the first organic compoundand the second organic compound is a π-electron deficient heteroaromaticcompound and the other is a π-electron rich heteroaromatic compound oran aromatic amine compound.
 14. The light-emitting device according toclaim 4, wherein an energy is transferred from a level of a singletexcited state of the thermally activated delayed fluorescent substanceto a level of a singlet excited state of the fluorescent material. 15.The light-emitting device according to claim 4, wherein a differencebetween the peak wavelength of the emission spectrum of the thermallyactivated delayed fluorescent substance and a peak wavelength of anemission spectrum of the fluorescent material is 30 nm or less.
 16. Thelight-emitting device according to claim 4, further comprising: a pairof electrodes; and an EL layer between the pair of electrodes, whereinthe EL layer comprises the light-emitting layer.